pan/mdg: Set the z/s store intrinsic base correctly

When EXT_shader_framebuffer_fetch is used and only depth and/or
stencil are written, we can't rely on the first output being to
depth/stencil.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5755>

src/panfrost/midgard/midgard_compile.c

@@ -401,7 +401,12 @@ midgard_nir_lower_zs_store(nir_shader *nir)
 
                         combined_store->num_components = 4;
 
-                        nir_intrinsic_set_base(combined_store, 0);
+                        unsigned base;
+                        if (z_store)
+                                base = nir_intrinsic_base(z_store);
+                        else
+                                base = nir_intrinsic_base(s_store);
+                        nir_intrinsic_set_base(combined_store, base);
 
                         unsigned writeout = 0;
                         if (z_store)