drm/msm/sde: allocate pingpong-id based on dsc-id

HW design expects to use pingpong-id same as dsc-id to
enable/disable dsc. The existing implementation uses
pingpong-id corresponding to layer-mixer to enable/disable
dsc. This implementation configures the wrong pingpong-id
when layer-mixer, pingpong and dsc module ids are not same.
This change allocates the pingpong module based on dsc and
uses this pingpong module id to enable/disable dsc.

Change-Id: Ie15447a3d46d9077590f316942dd2717eb5fa298
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>