msm: kgsl: Add kgsl_flush_event_group()

Add kgsl_flush_event_group() to deal with all the pending events in a
context.  Events with a retired timestamp are retired and still
pending events are canceled. This is intended for situations when
the context or event group is going away or will otherwise be
inoperable.

Change-Id: Ic0dedbadb308f95e4bf9097e8553e30d3ec2bdd4
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>