v3d: do not automatically flush current job for SSBOs and shader images

v3d: do not automatically flush current job for SSBOs and shader images

If the current job has a sequence of draw calls involving SSBOs and/or
shader images, we would flush the job in between each draw call.
With this change, we won't flush the current job and we rely on the
application inserting correct barriers by issuing glMemoryBarrier()
when needed.

v2 (Eric):
 - When mapping a buffer for writing, we always need to flush.

Reviewed-by: Eric Anholt <eric@anholt.net>