drm/vkms: Fix memory leak in vkms_init()

A memory leak was reported after the vkms module install failed.

unreferenced object 0xffff88810bc28520 (size 16):
  comm "modprobe", pid 9662, jiffies 4298009455 (age 42.590s)
  hex dump (first 16 bytes):
    01 01 00 64 81 88 ff ff 00 00 dc 0a 81 88 ff ff  ...d............
  backtrace:
    [<00000000e7561ff8>] kmalloc_trace+0x27/0x60
    [<000000000b1954a0>] 0xffffffffc45200a9
    [<00000000abbf1da0>] do_one_initcall+0xd0/0x4f0
    [<000000001505ee87>] do_init_module+0x1a4/0x680
    [<00000000958079ad>] load_module+0x6249/0x7110
    [<00000000117e4696>] __do_sys_finit_module+0x140/0x200
    [<00000000f74b12d2>] do_syscall_64+0x35/0x80
    [<000000008fc6fcde>] entry_SYSCALL_64_after_hwframe+0x46/0xb0

The reason is that the vkms_init() returns without checking the return
value of vkms_create(), and if the vkms_create() failed, the config
allocated at the beginning of vkms_init() is leaked.

 vkms_init()
   config = kmalloc(...) # config allocated
   ...
   return vkms_create() # vkms_create failed and config is leaked

Fix this problem by checking return value of vkms_create() and free the
config if error happened.

Fixes: 2df7af93

 ("drm/vkms: Add vkms_config type")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221101065156.41584-2-yuancan@huawei.com