drm/i915: overlay: drop superflous gpu flushes

Cache-coherency is maintained by gem. Drop these leftover MI_FLUSH
commands from the userspace code.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>