drm/msm/sde: add support for dsc with 3d-mux merge

Add support for dsc with 3d-mux merge topology that
uses 2 layer mixers, 1 dsc encoder and 1 interface.

Change-Id: I69979deea26b24028f5c3a7515e6408415f86e45
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>