i965: Allow only one slot of clip distances to be set on Gen4-5.

i965: Allow only one slot of clip distances to be set on Gen4-5.

The existing backend code assumed that if VARYING_SLOT_CLIP_DIST0
was written, then VARYING_SLOT_CLIP_DIST1 would be as well.  That's
true with the current lowering, but not necessary if there are 4 or
fewer clip distances.  Separate out the checks to allow this.

The new NIR-based lowering will trigger this case, which would have
caused backend validation errors (src is null) without this patch.

Reviewed-by: Eric Anholt <eric@anholt.net>