Add command buffer support.
Currently we don't implement preemption or fancy error handling.
Tested with a couple of mesa-demos, compiz/unity and viewperf maya-03.

v2:
- Synchronize with pending work at command buffer manager takedown.
- Add an interface to flush the current command buffer for latency-critical
  command batches and apply it to framebuffer dirtying.

v3:
- Minor fixes of definitions and typos to address reviews.
- Removed new or moved branch predictor hints.
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>