drm/msm: add new connector ops and mst cap in sde

Implement atomic_best_encoder and atomic_check ops
for sde connector and add a MST_MODE capability flag
that is useful in creating MST type drm encoders.

Change-Id: Ied7d9010df6835baeaf9aa4903c8a89ac662c78b
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>