msm: mdss: Create sysfs entry for PU state in MDP3

Creating a sysfs entry for PU state which will be used
for dynamically switching between display PP features
and PU.

state of sysfs entry:
0: PU disabled and PP enabled
1: PU enabled and PP disabled

Change-Id: If800db588339a3f2f346279c86edd841897a4481
Signed-off-by: Nitesh Gupta <niteshg@codeaurora.org>