disp: msm: sde: Register display brightness cooling device

Register for getting a blocking notified call when the display cooling
device mitigates. Update the current brightness to the thermally allowed
level.

Change-Id: I20945a20751646e6d9bbf8bb7e7039c6ce43c306
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>