v3d: fix glDrawTransformFeedback{Instanced}()

v3d: fix glDrawTransformFeedback{Instanced}()

This needs to take the vertex count from the provided transform
feedback buffer.

v2:
 - don't take the vertex count from the underlying buffer, instead,
   take it from a v3d subclass of pipe_stream_output_target (Eric).

Fixes piglit tests:
spec/ext_transform_feedback2/draw-auto
spec/ext_transform_feedback2/draw-auto instanced

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