msm: kgsl: Delete context event group on detach

Delete the context event group when the context is detached instead
of deleting it when context is destroyed. If the event group is deleted
when context is destroyed then a race occurs between deleting the event
group and processing the event group.

CRs-Fixed: 713905
Change-Id: Ifdc3397394d585f164f77d2e299a9d664bcbdc73
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>