msm: mdss: Create sysfs node for partial update state

Create a sysfs node for partial update state. This dyn_pu
sysfs node will be used to show whether partial update
feature is enabled or not, so that display post processing
features can be dynamically enable/disable accordingly.

Change-Id: I7012e7830575942299ae07c956eb27244e622365
Signed-off-by: Ping Li <pingli@codeaurora.org>
Signed-off-by: Nitesh Gupta <niteshg@codeaurora.org>
Signed-off-by: Bharat Pawar <bpawar@codeaurora.org>
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>