drm/msm/mdp: add common YUV information for MDP4/MDP5

Both MDP4 and MDP5 share some code as far as YUV support is
concerned. This change adds this information and will be followed
by the actual MDP4 and MDP5 YUV support patches.

Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>