msm: mdss: Notify userspace of idle screen case

Expose two new sysfs nodes - idle_time and idle_notify - that
are used to notify userspace whenever the display is idle with
no screen updates. The idle_time node needs to be configured
with the duration in milliseconds to detect no screen updates.
The userspace app can poll on the idle_notify node to get
notifications of the idle state.

Change-Id: I7435a084ad2c928d952c5d106ff9ef92aaa385b9
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>