disp: msm: sde: add support for AVR_STEP feature

Add AVR step support so SW can trigger a late frame and instead
of immediately triggering, HW will perform the update at the
start of the next step interval. This allows for a fixed SW
vsync timeline to be maintained in userland, eliminating the
usual drift from the actual HW vsync caused by a late frame.

This change adds AVR_STEP support via a DRM property.

Change-Id: I4cf8a296989805f134c2165a3bed0b050bb09c96
Signed-off-by: Steve Cohen <cohens@codeaurora.org>

msm/msm_drv.h

@@ -223,6 +223,7 @@ enum msm_mdp_conn_property {
 	CONNECTOR_PROP_QSYNC_MODE,
 	CONNECTOR_PROP_CMD_FRAME_TRIGGER_MODE,
 	CONNECTOR_PROP_SET_PANEL_MODE,
+	CONNECTOR_PROP_AVR_STEP,
 
 	/* total # of properties */
 	CONNECTOR_PROP_COUNT