drm/msm/dp: protect controller functionalities when powered off

DP controller exposes some of its functionalities to DP display.
DP display may call into these functionalities in response to
attention messages received from sink or user-space calls.
In some cases, DP controller may be in idle or powered off state.
Making calls to DP controller in such state may result in unstable
state. Check for DP controller state before calling its functionalities.

CRs-Fixed: 2163274
Change-Id: I0cd2653924e77ce9c673c1b3a3007fd200346314
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>