ir3: Fixup dual-source blending slot

The hardware expects that where MRT0 and MRT1 would normally go are the
dual sources for MRT0, whereas GLSL has an extra "index" parameter that
indicates which source it is. Remap it when handling FS outputs.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5039>

src/freedreno/ir3/ir3_compiler_nir.c

@@ -3112,6 +3112,7 @@ setup_output(struct ir3_context *ctx, nir_variable *out)
 			so->writes_smask = true;
 			break;
 		default:
+			slot += out->data.index; /* For dual-src blend */
 			if (slot >= FRAG_RESULT_DATA0)
 				break;
 			ir3_context_error(ctx, "unknown FS output name: %s\n",