drm/msm/dp: protect power on/off functionalities with mutex

In case of multiple threads trying to power on/off DP driver
simultaneously, un-clocked register access may happen. Use
session_lock in all conditions to avoid such scenario.

CRs-Fixed: 2381931
Change-Id: I6d8f82b3c37bf54965176dae50e0f9dca5343e2e
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>