i965: Always reserve clip distance VUE slots in SSO mode.

i965: Always reserve clip distance VUE slots in SSO mode.

This fixes rendering in Dolphin on Vulkan since we enabled clip
distances.  (Dolphin on GL has a similar bug because the linker
fails to eliminate unused clip distance built-in arrays, but it
isn't using SSO...so that needs more fixing.)

Also fixes a Piglit test:
spec/glsl-1.50/execution/geometry.clip-distance-vs-gs-out-sso

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>