drm/i915: Check for allocation failure

drm/i915: Check for allocation failure

The memory allocation for C is not being null checked and hence we
could end up with a null pointer dereference. Fix this with a null
pointer check. (I really should have noticed this when I was fixing an
earlier issue.)

Detected by CoverityScan, CID#1436406 ("Dereference null return")

Fixes: 47624cc3301b60 ("drm/i915: Import the kfence selftests for i915_sw_fence")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170519175617.7036-1-colin.king@canonical.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>