msm: kgsl: Panic if global timestamp wait fails in context detach

If we timeout while waiting for the last issued global timestamp for
a detaching context to expire (10 seconds!) then we are on a fast
train to crashville.  BUG_ON() so we can stand a chance of debugging
this very rare issue.

Change-Id: Ic0dedbaddb9cbc5681f9c872785ce26b7cf9b87b
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>