msm: kgsl: Improve the dispatcher issuecmds loop

Instead of playing silly tricks to try to avoid going into an
infinite loop while processing pending contexts do the smart
thing and copy off the entire pending queue into a temporary
list.  That leaves the master list free to accept new and
requeued contexts and we can do evil things to our temporary
list.

Change-Id: Ic0dedbad365206031854fc95b5353184cabd40a1
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>