msm: mdss: Turn off MDP GDSC in idle screen for cmd mode panels

For command mode panels, the DSI lanes can be configured to an
Ultra Low Power State (ULPS) during idle screen use case. In such
cases, it is possible to turn off the MDP GDSC as MDP is completely
idle. This would result in reducing any leakage current. Add
support for this feature.

Change-Id: Ic4074f9e12a27fe0fbe95219ffaa2f466abee307
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>