msm: mdss: add support to enable idle power collapse

Add support for a new configuration entry to enable mdss power
collapse for idle screen use-cases, when a command mode interface is
active. Currently idle power collapse happens only if the Ultra-Low
Power State feature of the DSI controller is enabled. Adding a separate
configuration entry provides the flexibility to enable this feature
independent of the state of the DSI controller.

Change-Id: I4732a95a9f3d0db3e7ecc96a36414349ac6b5604
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>