drm/msm/dp: add ch alloc/dealloc for mst simulator panel

Avoid the static channel allocation by implementing the
correct channel allocation based on the display mode for
streams in simulator panel mode.

CRs-Fixed: 2280286
Change-Id: Ie60a07a4f25fc45592ae5e8ec2b56de0ea9eb368
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>