intel/isl: Treat 3-D textures as 2-D arrays for rendering

intel/isl: Treat 3-D textures as 2-D arrays for rendering

In particular, this means that isl_view::base_array_layer and
isl_view::array_len get applied to 3-D textures but only when rendering.
We were already applying isl_view::base_array_layer for rendering into 3-D
textures so this isn't a huge deviation.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>