drm/msm/dp: enhancements in dp debug module

Enhance dp debug module to validate dp mst driver
in simulation mode. Implement debugfs nodes for
reading the mst support and mst active state info,
overriding mst connector status to simulate port
connect/disconnect event without real hot plug/unplug.

CRs-Fixed: 2332059
Change-Id: Idf43651825e30c676af6bf880aabeb752c5d0cc4
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>