drm/msm/mdp: mark if a MDP format is YUV at definition

This makes it easy to determine if a format is YUV. The old
method of using chroma sample type incorrectly marks YUV444 as
RGB format.

Signed-off-by: Wentao Xu <wentaox@codeaurora.org>
[rebase]
Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>