drm/msm/dp: add debugfs support to DP driver

To prepare the MSM DP driver for running video pattern
compliance tests introduce debugfs module for it.

Changes in v2: rebase on top of latest patchset of dependency

Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>