drm/msm/sde: properly disable DSC with ACTIVE CTL

The ACTIVE CTL present in recent HW revs requires some special
handling for disabling DSC. This patch includes the following
changes:
-Disconnect DSC binding from it's associated ping-pong
-Make sure DSC block is properly flushed in DSC disable path
-Clear the DSC ACTIVE bits in CTL

These changes are required to handle seamless mode switching
between DSC and non-DSC modes.

Change-Id: I76e18560240c293797275ea42df520ff9d816e15
Signed-off-by: Steve Cohen <cohens@codeaurora.org>