msm: mdss: use helper functions to check for panel power state

Whenever the display always-on feature is enabled, it is possible
that the display pipeline can be in an intermediate state between being
fully interactive to fully turned off. In the current implementation, a
number of state transition decisions are made based on whether the
display pipeline is on or not. Instead of explicitly checking for the
panel power state, define generic helper functions which classify the
panel power state into three different stages - interactive, low-power
and off. Replace any existing check for the panel power state with these
newly defined helper functions.

Change-Id: I4d6dd883ea6aca8abd57fa048a79e0c88da09a95
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>