drm/msm/sde: add mux control for lower pipe blocks

This change adds functionality to explicitly control the input
and output muxing from lower pipe components.  This change add
this feature to dsc, interfaces and the writeback component by
providing additional api to configure the pp block id.

Change-Id: I2cdfaae075cdd931286cdde1965d066e8e8df6fc
Signed-off-by: Steve Cohen <cohens@codeaurora.org>
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>