pan/mdg: Implement nir_intrinsic_load_sample_mask_in

Also goes down the magical ld_color_buffer_32u_old path, but with a
different magic number.

Maybe should just be the ld_magic instruction.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6516>