techpack: display: drm/msm: add idle state sysfs node

Add a sysfs mechanism to track the idle state of display subsystem.
This allows user space to poll on the idle state node to detect when
display goes idle for longer than the time set.

Bug: 139655049
Bug: 126304228
Change-Id: I21e3c7b0830a9695db9f65526c111ce5153d1764
Signed-off-by: Adrian Salido <salidoa@google.com>
Signed-off-by: Robb Glasser <rglasser@google.com>
(cherry picked from commit 11a2193b434cb3130743fbff89a161062883132e)
Signed-off-by: Ken Huang <kenbshuang@google.com>