drm/v3d: Refactor job management.

The CL submission had two jobs embedded in an exec struct.  When I
added TFU support, I had to replicate some of the exec stuff and some
of the job stuff.  As I went to add CSD, it became clear that actually
what was in exec should just be in the two CL jobs, and it would let
us share a lot more code between the 4 queues.

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