kgsl: convert some workqueues to use kthreads

adreno_dispatch_work and _kgsl_event_worker are both low-latency
low-runtime functions that are in the critical path of GPU
rendering. Moving them out of workqueues and into a dedicated FIFO
kthread avoids significant jitter.

bug 30342017
bug 62616267

Signed-off-by: Tim Murray <timmurray@google.com>
Change-Id: I5b071a738948b69c23810a8ea5957bdddc274912