drm/msm/dp: add support for simulation

Add debugfs node to enable/disable simulation mode.
Avoid AUX communication in simulation mode and override
it with debugfs data. This can be used for a quick
DP validation by triggering hot plug using debugfs nodes.

Change-Id: Id6a5112a4565feb15decf36b346c8cfd9783fd6f
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>