vc4: Fix handling of non-XYZW swizzles in color outputs.

The SWIZZLE_1 of the winsys destination was dereffing off the end of the
array, which surprisingly often worked out (since nobody reads the
rendered value anyway, so whatever junk was referenced in the QIR didn't
matter), but shader dumping would sometimes segfault.

src/gallium/drivers/vc4/vc4_program.c

@@ -967,12 +967,12 @@ emit_frag_end(struct tgsi_to_qir *trans)
                 blend_color[3] = qir_uniform_f(trans, 0.5);
         }
 
-        struct qreg swizzled_outputs[4] = {
-                blend_color[format_desc->swizzle[0]],
-                blend_color[format_desc->swizzle[1]],
-                blend_color[format_desc->swizzle[2]],
-                blend_color[format_desc->swizzle[3]],
-        };
+        struct qreg swizzled_outputs[4];
+        for (int i = 0; i < 4; i++) {
+                swizzled_outputs[i] =
+                        get_swizzled_channel(trans, blend_color,
+                                             format_desc->swizzle[i]);
+        }
 
         if (trans->fs_key->depth_enabled) {
                 qir_emit(c, qir_inst(QOP_TLB_PASSTHROUGH_Z_WRITE, c->undef,