drm/i915: Move the common engine cleanup to intel_engine_cs.c

drm/i915: Move the common engine cleanup to intel_engine_cs.c

Now that we initialize the state to both legacy and execlists inside
intel_engine_cs, we should also clean up that state from the common
functions.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470226756-24401-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>