pan/midgard: Add nir_intrinsic_store_zs_output_pan support

pan/midgard: Add nir_intrinsic_store_zs_output_pan support

ZS fragment stores are done like color fragment stores, except it's
using a different RT id (0xFF), the depth and stencil values are stored
in r1.x and r1.y.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
[Fix the scheduling part]

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