pan/midgard: Turn Z/S stores into zs_output_pan intrinsics

pan/midgard: Turn Z/S stores into zs_output_pan intrinsics

Midgard can't write depth and stencil separately. It has to happen in
a single store operation containing both. Let's add a panfrost specific
intrinsic and turn all depth/stencil stores into a packed depth+stencil
one.

Note that this intrinsic is not yet handled in emit_intrinsic(), but
we'll address that later.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3697>