msm:disp:dpu1: add mixer selection for display topology

mixer selection in the display topology is based on multiple
factors
1) mixers available in the hw
2) interfaces to be enabled
3) merge capability

change will pickup mixer as per the topology need.

Signed-off-by: Kalyan Thota <kalyan_t@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>