drm/msm: atomic seamless connector change

Handle dynamic connector attach or detach of additional
connector on crtc. Avoid full modeset disable and enable
if multiple connectors connected to single CRTC. It allows
seamless handling of usecase like concurrent writeback.
Attempt to keep existing seamless mode checks intact.

Change-Id: Ia8ab0ef27400f2ba08a7d2dc6984820f3c956231
Signed-off-by: Prabhanjan Kandula <pkandula@codeaurora.org>