v3d: fix TF primitive counts for resume without draw

v3d: fix TF primitive counts for resume without draw

The V3D documentation states that primitive counters are reset when
we emit Tile Binning Mode Configuration items, which we do at the start
of each draw call, however, in the actual hardware this doesn't seem to
take effect when transform feedback is not active (this doesn't happen in
the simulator). This causes a problem in the following scenario:

glBeginTransformFeedback()
   glDrawArrays()
   glPauseTransformFeedback()
   glDrawArrays()
   glResumeTransformFeedback()
glEndTransformFeedback()

The TF pause will trigger a flush of the primitive counters, which results
in a correct number of primitives up to that point. In theory, the counter
should then be reset when we execute the draw after pausing TF, but that
doesn't happen, and since TF is enabled again by the resume command before
we end recording, by the time we end the transform feedback recording we
again check the counters, but instead of reading 0, we read again the same
value we read at the time we paused, incorrectly accumulating that value
again.

In theory, we should be able to avoid this by using the other method to
reset the primitive counters: using operation 1 instead of 0 when we
flush the counts to the buffer at the time we pause, but again, this
doesn't seem to be work and we still see obsolete counts by the time we
end transform feedback.

This patch fixes the problem by not accumulating TF primitive counts
unless we know we have actually queued draw calls during transform
feedback, since that seems to effectively reset the counters. This should
also be more performant, since it saves unnecessary stalls for the
primitive counters to be updated when we know there haven't been any
new primitives drawn.

Fixes CTS tests:
dEQP-GLES3.functional.transform_feedback.*

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