msm: mdss: add idle state node

Add a helper node that can be used to notify user space through sysfs
node when fb device has not had any activity for a specified amount of
time (through idle_time node).

Bug: 62110101
Change-Id: I4dfa4b1a376149aa55a940dad7ac336ec99f1af8
Signed-off-by: Adrian Salido <salidoa@google.com>