anv/pipeline: use less instructions for multiview

anv/pipeline: use less instructions for multiview

The view_index is encoded in the remainder of dividing instance id by
the number of views in the view mask (n). In the general case (handled
by the else clause), there is a need to map from 0..n-1 into the
number of the view being masked. For that a map is encoded.

In the case only the first n bits in the mask are set, the mapping is
trivial, 0..n-1 already represent what view is being referred to.

That case was in the original patch that added
anv_nir_lower_multiview.c but disabled.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>