drm/i915: don't reference null pointer at i915_sink_crc

drm/i915: don't reference null pointer at i915_sink_crc

Reproducible by runtime suspending a Haswell machine with eDP + HDMI
outputs connected.

[  209.600086] [drm:i915_runtime_suspend], Suspending device
[  209.688435] BUG: unable to handle kernel NULL pointer dereference at 0000000000000060
[  209.688500] IP: [<ffffffffa0109d4e>] i915_sink_crc+0x6e/0xf0 [i915]
[  209.688577] PGD 36aba067 PUD 35d7f067 PMD 0
[  209.688613] Oops: 0000 [#1] SMP
[  209.688641] Modules linked in: fuse ip6table_filter ip6_tables ebtable_nat ebtables iTCO_wdt iTCO_vendor_support x86_pkg_temp_thermal coretemp microcode serio_raw e1000e pcspkr i2c_i801 ptp mei_me mei lpc_ich mfd_core pps_core dm_crypt i915 i2c_algo_bit crc32_pclmul drm_kms_helper crc32c_intel drm ghash_clmulni_intel video
[  209.688893] CPU: 1 PID: 1797 Comm: pm_pc8 Not tainted 3.13.0+ #118
[  209.688937] Hardware name: Intel Corporation Shark Bay Client platform/WhiteTip Mountain 1, BIOS HSWLPTU1.86C.0133.R00.1309172123 09/17/2013
[  209.689023] task: ffff88007fb4b690 ti: ffff88007d9d2000 task.ti: ffff88007d9d2000
[  209.689074] RIP: 0010:[<ffffffffa0109d4e>]  [<ffffffffa0109d4e>] i915_sink_crc+0x6e/0xf0 [i915]
[  209.689169] RSP: 0018:ffff88007d9d3e68  EFLAGS: 00010246
[  209.689205] RAX: 0000000000000000 RBX: ffff880036a03478 RCX: ffff8800366c9770
[  209.689252] RDX: ffff88014325cf38 RSI: ffff88007fb4bd08 RDI: ffff88007fb4b690
[  209.689299] RBP: ffff88007d9d3e98 R08: 0000000000000000 R09: 0000000000000000
[  209.689346] R10: 0000000000000001 R11: 0000000000000000 R12: ffff8800366c9148
[  209.689393] R13: 00000000ffffffed R14: ffff88007d9d3f50 R15: ffff880036a03478
[  209.689441] FS:  00007f5a74bc29c0(0000) GS:ffff88014f240000(0000) knlGS:0000000000000000
[  209.689494] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  209.689533] CR2: 0000000000000060 CR3: 0000000079d7e000 CR4: 00000000001407e0
[  209.689580] Stack:
[  209.689594]  0000000000001000 ffff880146083980 ffff880146083980 0000000000000000
[  209.689649]  ffff880146083980 0000000000000001 ffff88007d9d3f00 ffffffff811d0744
[  209.689702]  0000000000000046 00007fff7949fe20 ffff880036a034b8 0000000000000080
[  209.689756] Call Trace:
[  209.689778]  [<ffffffff811d0744>] seq_read+0x164/0x3e0
[  209.689816]  [<ffffffff811ab165>] vfs_read+0x95/0x160
[  209.689851]  [<ffffffff811abc79>] SyS_read+0x49/0xa0
[  209.689888]  [<ffffffff810ef64c>] ? __audit_syscall_entry+0x9c/0xf0
[  209.689933]  [<ffffffff81659412>] system_call_fastpath+0x16/0x1b

Testcase: igt/pm_pc8 (do a full run, it will fail at the debugfs-read subtest)
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Flip around NULL check for robustness.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>