radeonsi: fix gl_ClipDistance and gl_ClipVertex for points

radeonsi: fix gl_ClipDistance and gl_ClipVertex for points

The clipper hardware doesn't consider points as primitives that can be
clipped. Simply setting the corresponding cull bits works, and should not
have an adverse effect on other primitive types according to the hardware
team.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>