msm: mdss: dp: fix wdog_bite while disconnecting DP cable

As a corner case, if DISCONNECT_IRQ_HPD event is timedout and
now if cable is disconnected and userspace triggers BLANK call
at the same time, dp core may be turned off before trying to
put it in IDLE state. Add changes to avoid this un-clocked
register access.

Change-Id: I3511443a3be4d3b70f5e68b31234b682e475ebdf
Signed-off-by: Narender Ankam <nankam@codeaurora.org>