drm/i915: Use a local to shorten req->i915->gpu_error.wait_queue

Use a local variable to avoid having to type out the full name of the
gpu_error wait_queue.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170223074422.4125-5-chris@chris-wilson.co.uk