framebuffer: add a pollable sysfs entry for display status

Wakeup the userspace poll on change of display status. The
userspace then may take action to change the
power/performance characteristics of the device.

Change-Id: I9bd11f3d895d3d83230104f5d8dcd1deeaa754cd
Signed-off-by: Amar Singhal <asinghal@codeaurora.org>