drm/i915: Remove unused variable from execlists_context_queue

After commit d7b9ca2f
("drm/i915: Remove request->uniq")

dev_priv is no longer needed.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>