If the clipped dirty region's x/y happened to align to 256, we would
have set the top 8 bits wrong.  Noticed by inspection, not by
reproducing a bug.
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181024184313.2967-4-eric@anholt.netReviewed-by: Noralf Trønnes <noralf@tronnes.org>