msm: mdss: refactor MDSS idle power collapse logic

MDSS GDSC can be turned off, or power-collased for command mode panels
in static screen use-cases. In the current implementation, the power
collapse is triggered by a worker thread in the interface driver
whenever once it enters the idle state. However, MDP driver already
registers pm_runtime callbacks for idle and suspend notifications and
also maintains a count of active overlay sessions. It is much simpler
and cleaner to trigger idle-power collapse based on that. This also
avoids unnecessary synchronization issues pertaining to the power
collapse worker thread.

Change-Id: I96ee339b96ad794b2d87d19d1c14768fca3fa192
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>