v3d: emit 'Wait for transform feedback' commands when needed

v3d: emit 'Wait for transform feedback' commands when needed

The hardware can flush transform feedback writes before reads in the same
job by inserting this command.

This patch detects when the rendering state for the current draw call reads
resources that had been previously written by transform feedback in the
same job and inserts the 'Wait for transform feedback' command before
emitting the new draw.

v2 (Eric):
  - this was intended to look at job->tf_write_prscs for TF jobs.
  - clear job->tf_write_prscs after we emit the TF flush.
  - can skip flushes for fragment shader reads from TF.

v3 (Eric):
  - all resources in job->tf_write_prscs are resources written by TF so
   we don't need to check if they are bound to PIPE_BIND_STREAM_OUTPUT.
  - documented optimization opportunity for geometry stages.

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