drm/i915: Check rq->timeline before deference

Not only is the context suspect to disappearing, but so is it's
timeline. Under a lockless inspection of the requests for
debugging from intel_engine_dump(), the context may already have been
freed and we have to check before chasing the dangling pointer.

[28033.681755] Modules linked in: vgem snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic x86_pkg_temp_thermal intel_powerclamp coretemp snd_hda_intel crct10dif_pclmul crc32_pclmul snd_hda_codec snd_hwdep snd_hda_core ghash_clmulni_intel snd_pcm mei_me mei i915 r8169 mii prime_numbers i2c_hid
[28033.681796] CPU: 3 PID: 3058 Comm: gem_exec_schedu Tainted: G     U           4.16.0-rc5+ #9
[28033.681804] Hardware name: Acer Aspire E5-575G/Ironman_SK  , BIOS V1.12 08/02/2016
[28033.681834] RIP: 0010:print_request+0x2b/0xb0 [i915]
[28033.681840] RSP: 0018:ffffc90004afbc18 EFLAGS: 00010202
[28033.681847] RAX: 6b6b6b6b6b6b6b6b RBX: ffff8801921b5a40 RCX: 0000000000000006
[28033.681854] RDX: ffffc90004afbc60 RSI: ffff8801921b5a40 RDI: 0000000000000004
[28033.681861] RBP: ffffc90004afbd80 R08: 0000000000000000 R09: 0000000000000001
[28033.681868] R10: ffffc90004afbbd0 R11: ffffc90004afbc73 R12: ffffc90004afbc60
[28033.681875] R13: ffffc90004afbd80 R14: ffff8801d40ec670 R15: ffff8801921b5a40
[28033.681883] FS:  00007fbba5f6c8c0(0000) GS:ffff8801e8400000(0000) knlGS:0000000000000000
[28033.681891] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[28033.681897] CR2: 00007fbba5f8f000 CR3: 00000001b2efa002 CR4: 00000000003606e0
[28033.681904] Call Trace:
[28033.681932]  intel_engine_print_registers+0x6a7/0x930 [i915]
[28033.681962]  intel_engine_dump+0x30d/0x740 [i915]
[28033.681971]  ? seq_printf+0x3a/0x50
[28033.681995]  i915_engine_info+0xb8/0xe0 [i915]
[28033.682003]  ? drm_get_color_range_name+0x20/0x20
[28033.682010]  seq_read+0xe1/0x440
[28033.682018]  full_proxy_read+0x51/0x80
[28033.682025]  __vfs_read+0x21/0x130
[28033.682031]  ? do_sys_open+0x134/0x220
[28033.682037]  ? kmem_cache_free+0x177/0x2b0
[28033.682043]  vfs_read+0xa1/0x150
[28033.682049]  SyS_read+0x40/0xa0
[28033.682055]  do_syscall_64+0x6b/0x1b0
[28033.682063]  entry_SYSCALL_64_after_hwframe+0x42/0xb7
[28033.682069] RIP: 0033:0x7fbba4655d11
[28033.682074] RSP: 002b:00007ffd8c49da58 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
[28033.682082] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fbba4655d11
[28033.682089] RDX: 000000000000003f RSI: 00005647bfbfc260 RDI: 0000000000000006
[28033.682096] RBP: 000000000000003f R08: 00000000ffffffff R09: 0000000000000000
[28033.682104] R10: 0000000000000000 R11: 0000000000000246 R12: 00005647bfbfc260
[28033.682111] R13: 0000000000000006 R14: 0000000000000000 R15: 00005647bfbfc260
[28033.682119] Code: 41 55 41 54 49 89 d4 55 53 48 89 fd 48 8b 86 c8 00 00 00 48 8b 3d d6 1e 14 e2 48 89 f3 48 2b be a8 02 00 00 48 8b 80 b0 00 00 00 <4c> 8b 68 18 e8 bc 80 02 e1 8b 8b 70 02 00 00 8b b3 28 02 00 00
[28033.682206] RIP: print_request+0x2b/0xb0 [i915] RSP: ffffc90004afbc18

Reported-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180314101630.8933-1-chris@chris-wilson.co.uk