msm: kgsl: Check long IB only for valid context

Do not check for long IB if the hang is detected in a NULL context,
hang in NULL context can happen when we are switching from NULL to
active context and hang is detected in commands that write an
active context to memstore, long IB in this scenario is not possible.

Change-Id: I7711c86b91c6c2ce0b4aa0543572c26d10c83030
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>