a3xx: make use of software clipping when hw can't handle it

a3xx: make use of software clipping when hw can't handle it

The hw clipper only handles up to 6 UCPs. If there are more than 6 UCPs,
or a clip vertex, or clip distances are in use, then we must use the
fallback discard-based clipping from the frag shader.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org