drm: msm: add support for ltm off event

Clients of local tone mapping engine need to know when hardware block
was turned off, to enable mutually exclusively display features.
Change adds support for ltm off event notification via custom event
interface of drm.

Change-Id: Ibfe2f85eadb0b939deee56194387b51b1e5ca8b9
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>