Using fb modifier flag, support NV12MT format in MDP4.

v2:
- rework the modifier's description [Daniel Vetter's comment]
- drop .set_mode_config() callback [Rob Clark's comment]
v3:
- change VENDOR's name and restrict usage to NV12 [pointed by Daniel]
Signed-off-by: NRob Clark <robdclark@gmail.com>