drm/msm/sde: set the qsync/avr mode during encoder enable

Set the Qsync AVR control mode register during encoder enable.
This ensures that the state of this feature matches the connector
property setting after suspend/resume events.

Change-Id: I8c8d2b2d3cd2729f3c398189476ad79979302de9
Signed-off-by: Steve Cohen <cohens@codeaurora.org>