msm: kgsl: Do not allocate space in ringbuffer for hung context

If a hang is detected when allocating space in ringbuffer and
if the context for which the space is being allocated is hung
then do not allocate space at all.

Change-Id: Ia5ade2341fe5016119d8c140413860420c5c3a3d
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>