drm/msm/sde: Enable dirty bits for const alpha change

Enable dirty plane and dirty rects bits in case of
const alpha change in planes. If not, corruption might
be observed in this case.

Change-Id: I82e1786fabbca9be2cb6e1eb1aa270a45e257b68
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>