nir: Add texture sources and intrinsics for bindless

nir: Add texture sources and intrinsics for bindless

On Intel, we have both bindless and bindful and we'd like to use them at
the same time if we can so we need to be able to distinguish at the NIR
level between the two.  This also fixes nir_lower_tex to properly handle
bindless in its tex_texture_size and get_texture_lod helpers.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>