drm/i915/overlay: Ensure that the reg_bo is in the GTT prior to writing.

Just makes sure that writes are not being aliased by the CPU cache and
do make it out to main memory.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=24977
Cc: stable@kernel.org