drm/i915: Use ring->flush() instead of MI_FLUSH

Use the ring abstraction to hide the details of having choose the
appropriate flushing method.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

diff --cc drivers/gpu/drm/i915/i915_drv.h
Simple merge

diff --cc drivers/gpu/drm/i915/i915_gem.c
Simple merge

diff --cc drivers/gpu/drm/i915/intel_display.c
Simple merge