msm: kgsl: Flush the event workqueue if no contexts are available

If there are no open context IDs available for a new context try
flushing the event workqueue to process any outstanding
events that might be holding up detached contexts and then try
again one more time.

Change-Id: Ic0dedbadb875910fe32ce0f880152f47cf3404c1
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>