ac/nir: Add shader support for multiviews.

ac/nir: Add shader support for multiviews.

It uses an user SGPR to pass the view index to the shaders, except
for the fragment shader where we use layer=view (which comes in
handy when we want to do the NV ext that allows us to execute pre-FS
stages once instead of per view).

Reviewed-by: Dave Airlie <airlied@redhat.com>