disp: msm: dp: unify hpd event for sst and mst

Currently hpd uevent notification to usermode is triggered at different
points in the hpd handler callflow. With respect to the SST callflow,
the MST callflow has the following issues:
* the completion event object is getting reinitialized after the uevent
is sent
* The NOTIFIED states are not updated properly.
* dp_display_process_mst_hpd_high is overloaded to do two different
functions in the same flow and is controlled by the mst_probe argument.

This change cleans up the hpd callflows for MST and unifies the hpd
event notification. Also moved the mst check logic from
dp_display_process_mst_hpd_high to a separate function.

Change-Id: I8fdc92d2f9aae16d248c74643cb93688786dfbd5
Signed-off-by: Rajkumar Subbiah <rsubbia@codeaurora.org>