radeonsi: Don't leave gaps between position exports from vertex shader

radeonsi: Don't leave gaps between position exports from vertex shader

If the vertex shader exports clip distances but not point size, use
position exports 1/2 instead of 2/3 for the clip distances. Fixes
geometry corruption in that case.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66974

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>