pan/mdg: Add new depth store lowering

pan/mdg: Add new depth store lowering

This uses the new nir_intrinsic_store_combined_output_pan intrinsic,
which can write depth, stencil and color in a single instruction. If
there are no color writes, the "depth RT" is written to.

Fixes the dEQP GLES3 depth write tests, as well as the piglit tests
fragdepth_gles2, glsl-1.10-fragdepth and when modified to not rely
on depth/stencil reload, glsl-fs-shader-stencil-export.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5065>