disp: msm: dp: avoid use of mst_lock in hpd callbacks

DP MST driver registers bridge callback functions with the DRM framework.
mst_lock is used for synchronization of the mst bridge functions. These
functions call into dp_display where a session lock is obtained for
the display. HPD callbacks, on the other hand, are first handled by
dp_display. During these callbacks, dp_display takes a session lock
first before calling into DP MST driver layer. Currently, the DP MST
layer waits on mst_lock which can result in a deadlock.

This change removes the usage of mst_lock inside hpd callback functions
and just uses the session lock for synchronization.

Change-Id: I7ec338004eef43ac0f1e1f092463e9915cd60684
Signed-off-by: Rajkumar Subbiah <rsubbia@codeaurora.org>