drm/i915: Add execution priority boosting for mmioflips

drm/i915: Add execution priority boosting for mmioflips

Commit 6b5e90f58c56 ("drm/i915/scheduler: Boost priorities for flips")
added priority boosting for the modern atomic pageflips (and modesets),
but we should do the same for existing users of mmioflips (we don't yet
need to consider csflips as they are not used by execlists and so do not
have any support for a scheduler).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20161115092249.18356-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>