drm/v3d: Free the job and assign it to NULL if initialization fails

Currently, if `v3d_job_init()` fails (e.g. in the IGT test "bad-in-sync",
where we submit an invalid in-sync to the IOCTL), then we end up with
the following NULL pointer dereference:

[   34.146279] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000078
[   34.146301] Mem abort info:
[   34.146306]   ESR = 0x0000000096000005
[   34.146315]   EC = 0x25: DABT (current EL), IL = 32 bits
[   34.146322]   SET = 0, FnV = 0
[   34.146328]   EA = 0, S1PTW = 0
[   34.146334]   FSC = 0x05: level 1 translation fault
[   34.146340] Data abort info:
[   34.146345]   ISV = 0, ISS = 0x00000005, ISS2 = 0x00000000
[   34.146351]   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[   34.146357]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[   34.146366] user pgtable: 4k pages, 39-bit VAs, pgdp=00000001232e6000
[   34.146375] [0000000000000078] pgd=0000000000000000, p4d=0000000000000000, pud=0000000000000000
[   34.146399] Internal error: Oops: 0000000096000005 [#1] PREEMPT SMP
[   34.146406] Modules linked in: rfcomm snd_seq_dummy snd_hrtimer snd_seq snd_seq_device algif_hash aes_neon_bs aes_neon_blk algif_skcipher af_alg bnep hid_logitech_hidpp brcmfmac_wcc brcmfmac brcmutil hci_uart vc4 btbcm cfg80211 bluetooth bcm2835_v4l2(C) snd_soc_hdmi_codec binfmt_misc cec drm_display_helper hid_logitech_dj bcm2835_mmal_vchiq(C) drm_dma_helper drm_kms_helper videobuf2_v4l2 raspberrypi_hwmon ecdh_generic videobuf2_vmalloc videobuf2_memops ecc videobuf2_common rfkill videodev libaes snd_soc_core dwc2 i2c_brcmstb snd_pcm_dmaengine snd_bcm2835(C) i2c_bcm2835 pwm_bcm2835 snd_pcm mc v3d snd_timer snd gpu_sched drm_shmem_helper nvmem_rmem uio_pdrv_genirq uio i2c_dev drm fuse dm_mod drm_panel_orientation_quirks backlight configfs ip_tables x_tables ipv6
[   34.146556] CPU: 1 PID: 1890 Comm: v3d_submit_csd Tainted: G         C         6.7.0-rc3-g49ddab089611 #68
[   34.146563] Hardware name: Raspberry Pi 4 Model B Rev 1.5 (DT)
[   34.146569] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[   34.146575] pc : drm_sched_job_cleanup+0x3c/0x190 [gpu_sched]
[   34.146611] lr : v3d_submit_csd_ioctl+0x1b4/0x460 [v3d]
[   34.146653] sp : ffffffc083cbbb80
[   34.146658] x29: ffffffc083cbbb90 x28: ffffff81035afc00 x27: ffffffe77a641168
[   34.146668] x26: ffffff81056a8000 x25: 0000000000000058 x24: 0000000000000000
[   34.146677] x23: ffffff81065e2000 x22: ffffff81035afe00 x21: ffffffc083cbbcf0
[   34.146686] x20: ffffff81035afe00 x19: 00000000ffffffea x18: 0000000000000000
[   34.146694] x17: 0000000000000000 x16: ffffffe7989e34b0 x15: 0000000000000000
[   34.146703] x14: 0000000004000004 x13: ffffff81035afe80 x12: ffffffc083cb8000
[   34.146711] x11: cc57e05dfbe5ef00 x10: cc57e05dfbe5ef00 x9 : ffffffe77a64131c
[   34.146719] x8 : 0000000000000000 x7 : 0000000000000000 x6 : 000000000000003f
[   34.146727] x5 : 0000000000000040 x4 : ffffff81fefb03f0 x3 : ffffffc083cbba40
[   34.146736] x2 : ffffff81056a8000 x1 : ffffffe7989e35e8 x0 : 0000000000000000
[   34.146745] Call trace:
[   34.146748]  drm_sched_job_cleanup+0x3c/0x190 [gpu_sched]
[   34.146768]  v3d_submit_csd_ioctl+0x1b4/0x460 [v3d]
[   34.146791]  drm_ioctl_kernel+0xe0/0x120 [drm]
[   34.147029]  drm_ioctl+0x264/0x408 [drm]
[   34.147135]  __arm64_sys_ioctl+0x9c/0xe0
[   34.147152]  invoke_syscall+0x4c/0x118
[   34.147162]  el0_svc_common+0xb8/0xf0
[   34.147168]  do_el0_svc+0x28/0x40
[   34.147174]  el0_svc+0x38/0x88
[   34.147184]  el0t_64_sync_handler+0x84/0x100
[   34.147191]  el0t_64_sync+0x190/0x198
[   34.147201] Code: aa0003f4 f90007e8 f9401008 aa0803e0 (b8478c09)
[   34.147210] ---[ end trace 0000000000000000 ]---

This happens because we are calling `drm_sched_job_cleanup()` twice:
once at `v3d_job_init()` and again when we call `v3d_job_cleanup()`.

To mitigate this issue, we can return to the same approach that we used
to use before 464c61e7: deallocate the job after `v3d_job_init()`
fails and assign it to NULL. Then, when we call `v3d_job_cleanup()`, job
is NULL and the function returns.

Fixes: 464c61e7 ("drm/v3d: Decouple job allocation from job initiation")
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240109142857.1122704-1-mcanal@igalia.com