drm/i915: Release and unlock on mmap_gtt error path.

We failed to unlock the mutex after failing to create the mmap offset.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>
1 file changed