drm/msm/sde: add support for qsync video mode

Add support to enable qsync/AVR feature for video mode panels.
This feature allows video mode panels to update the refresh
rate anywhere from default (60fps) down to the slowest rate
supported by the panel.  This helps to reduce frame drops when
the commit arrives a little late but still within the panel's
supported refresh rate range.

Change-Id: I6546d0663ed7962f0c576d100c539038843bf4b1
Signed-off-by: Steve Cohen <cohens@codeaurora.org>