msm: kgsl: create singlethread wq instead of per-cpu wq

There doesn't seem to be a need for a per-CPU workqueue - kgsl can
make do with a singlethread workqueue which consumes less resources
and does not pin work to specific CPUs.

Change-Id: I6f9bd368434797a05497be88c24a5b1645d2f221
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>