drm/msm/sde: Fix disabling of Merge3D block

Fix disabling of Merge3D during a seamless mode switch.  When
switching from Merge3D to DSC merge, the register programming
to disable Merge3D must be flushed in the same vsync as the DSC
enable.  Also ensure Merge3D ops are populated only for HW that
supports this feature.

Change-Id: I0c4ae477db1b0d651cc35ae6cc70548eb9a474e6
Signed-off-by: Steve Cohen <cohens@codeaurora.org>