nir: Add new 'plane' texture source type

This will be used to select the plane to sample from for planar
textures.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir.h

@@ -1074,6 +1074,7 @@ typedef enum {
    nir_tex_src_ddy,
    nir_tex_src_texture_offset, /* < dynamically uniform indirect offset */
    nir_tex_src_sampler_offset, /* < dynamically uniform indirect offset */
+   nir_tex_src_plane,          /* < selects plane for planar textures */
    nir_num_tex_src_types
 } nir_tex_src_type;