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.

[myfluxi: Fix-up for hammerhead]

Change-Id: Ic4074f9e12a27fe0fbe95219ffaa2f466abee307
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>
msm: mdss: hold mdp clock ref count when restoring ctl path

MDP control path needs to be restored whenever MDSS GDSC is turned
back on while exiting the utlra-low power state (ULPS). Hold a
reference count for the MDP clocks to ensure that there is no
unclocked register access.

Change-Id: Ia2d299d9db9c23e4c9171cbec8e376908046494b
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>