msm: kgsl: Don't block on detach if the context is invalid

An invalidated context means that something pretty nasty happened and
likely there was an unrecovered GPU fault along the way. If this is
the case then the GPU is likely not processing commands for the context
anymore. Don't block for the last retired global if the context was
invalidated.

Change-Id: Ic0dedbad22a22b704baf14557d7e18cc4cf1044a
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>