msm: mdss: dp: fix no display issues during cable plugin/plugouts

If DP cable is disconnected while processing HPD or IRQ_HPD,
DP driver may continue with failsafe parameters and notify
connection event and immediately notify disconnection event
which may result in state machine corruption in userspace.
Add changes to avoid reading dpcd caps, edid, link training
or connection events if DP cable is disconnected in between.

Change-Id: I0b59ebdb636c9dc1086673253399b849734d51ee
Signed-off-by: Narender Ankam <nankam@codeaurora.org>