msm: mdss: Move PP reg bus vote to reg bus voting framework

There is a new framework for MDSS voting on the reg bus. However,
the postprocessing client was never registered, and therefore making
bus requests without the framework. This change moves the PP reg bus
voting to be a part of the MDSS bus voting framework.

Change-Id: I054b102baec0ccae56e7b9faafa8feadf3fc8d7a
Signed-off-by: Benet Clark <benetc@codeaurora.org>