i965/fs_surface_builder: Explicitly handle FORMAT_NONE in num_image_coordinates

Previously, we were relying on has_matching_typed_format returning true for
MESA_FORMAT_NONE which, in turn, relied on _mesa_get_format_bytes returning
1 for MESA_FORMAT_NONE.  When we switch to ISL, this behaviour will no
longer be something we can rely on.

Reviewed-by: Chad Versace <chad.versace@intel.com>

src/mesa/drivers/dri/i965/brw_fs_surface_builder.cpp

@@ -410,6 +410,7 @@ namespace {
           * reads want the array index to be at the Z component.
           */
          const bool array_index_at_z =
+            format != MESA_FORMAT_NONE &&
             !image_format_info::has_matching_typed_format(
                bld.shader->devinfo, format);
          const unsigned zero_dims =