radv: fix begin/end transform feedback with 0 counter buffers.

radv: fix begin/end transform feedback with 0 counter buffers.

If the user gives 0 counterBuffers then the driver should still
enable transform feedback on all targets. This changes the
driver to always enable xfb, and use counter buffers where
one is defined for the target in question.

Fixes: b4eb029062 (radv: implement VK_EXT_transform_feedback)
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 677b496b6bd07cbe05dd429344ba525619cdd08c)