panfrost: Fix dual-source blending

If dual blending is enabled, only 1 output is supported. Multiple
outputs confuse the write combining pass in this case, leading to
incorrect output and/or an assert failure in emit_fragment_store.

The fix is straightforward, just skip the speculative emitting of
multiple outputs in the case where dual source blending is enabled.

This also adds an extra sanity check in `pan_nir_lower_zs_store` to
check for only one blend store being present.

Fixes: c65a9be421 ("panfrost: Preprocess shaders at CSO create time")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/9487
Co-Authored-By: Eric R. Smith <eric.smith@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26474>

src/gallium/drivers/panfrost/pan_shader.c

@@ -405,7 +405,7 @@ panfrost_create_shader_state(struct pipe_context *pctx,
    if (nir->info.stage == MESA_SHADER_FRAGMENT &&
        nir->info.outputs_written & BITFIELD_BIT(FRAG_RESULT_COLOR)) {
 
-      NIR_PASS_V(nir, nir_lower_fragcolor, 8);
+      NIR_PASS_V(nir, nir_lower_fragcolor, nir->info.fs.color_is_dual_source ? 1 : 8);
       so->fragcolor_lowered = true;
    }
 

src/panfrost/util/pan_lower_writeout.c

@@ -106,6 +106,7 @@ pan_nir_lower_zs_store(nir_shader *nir)
                stores[1] = intr;
                writeout |= PAN_WRITEOUT_S;
             } else if (sem.dual_source_blend_index) {
+               assert(!stores[2]); /* there should be only 1 source for dual blending */
                stores[2] = intr;
                writeout |= PAN_WRITEOUT_2;
             }