msm: kgsl: Add timestamp events

Add support for triggering asynchronous events when a timestamp
expires.  The infrastructure is generic enough to support different
sorts of events and callbacks.  The first user of the event
infrastructure is a way to release a genlock locks on behalf of a
user space process.

Change-Id: Ic0dedbadfe67d98a5678453cbe0ac6996ba5c68f
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>