drm/i915/lrc: Update function names to match request flow

With adding engine->submit_request, we now have a bunch of functions
with similar names used at different stages of the execlist submission.
Try a different coat of paint, to hopefully reduce confusion between the
requests, intel_engine_cs and the actual execlists submision process.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-24-git-send-email-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470174640-18242-15-git-send-email-chris@chris-wilson.co.uk