drm/msm/dsi-staging: add support for command mode panels

Add support for command mode panels. Command engine state must be
ref-counted since it is toggled in two paths, command transfer and mdp
command frame updates.

Change-Id: I9aba97465ad822c4c862f6aa5422dc47cb6664c8
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>