mesa: set a type for depth fallback texture

Otherwise we will later attempt to figure out format with type GL_NONE which is not handled by _mesa_format_from_format_and_type.

Fixes: 0c6e56c3 ("mesa: (more) correctly handle incomplete depth textures") Closes: #9012 (closed) Signed-off-by: Tapani Pälli tapani.palli@intel.com