radeonsi: Don't export unused clip distance vectors from vertex shader

radeonsi: Don't export unused clip distance vectors from vertex shader

E.g. the Source engine seems to always write to gl_ClipVertex, but normally
doesn't enable any GL_CLIP_DISTANCEn states. This change removes some
irrelevant parts from the generated vertex shader code in such cases.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>