msm: mdss: add support for display always on mode

Add a new panel power state to represent a panel low power mode. Add a
new sysfs node to control when to configure the panel into this new
mode.

Change-Id: I7df8bf63470bec893c6b7481678f00ce76b6875c
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>