i965: write layer if gl_Layer is used in VS

This is enabled by the AMD_vertex_shader_layer extension.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp

@@ -2597,6 +2597,10 @@ vec4_visitor::emit_psiz_and_flags(struct brw_reg reg)
          emit(MOV(brw_writemask(reg, WRITEMASK_W),
                   src_reg(output_reg[VARYING_SLOT_PSIZ])));
       }
+      if (prog_data->vue_map.slots_valid & VARYING_BIT_LAYER) {
+         emit(MOV(retype(brw_writemask(reg, WRITEMASK_Y), BRW_REGISTER_TYPE_D),
+                  src_reg(output_reg[VARYING_SLOT_LAYER])));
+      }
    }
 }