v3d: Respect nir_intrinsic_store_output's write_mask.

Usually lower_io_to_temps sorts this out for us so you only get full
writes, but we should be able to handle it without that.  Avoids a
regression with the mesa/st PBO VS with layer output.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23114>

src/broadcom/compiler/v3d_nir_lower_io.c

@@ -223,6 +223,8 @@ v3d_nir_lower_vpm_output(struct v3d_compile *c, nir_builder *b,
                 int vpm_offset =
                         v3d_varying_slot_vpm_offset(c, location, start_comp + i);
 
+                if (!(nir_intrinsic_write_mask(intr) & (1 << i)))
+                        continue;
 
                 if (vpm_offset == -1)
                         continue;