st/mesa: set result writemask based on ir type

This prevents textureQueryLevels, which maps as LODQ, from ending up
with a xyzw writemask, which is illegal.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100061
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/state_tracker/st_glsl_to_tgsi.cpp

@@ -4166,6 +4166,7 @@ glsl_to_tgsi_visitor::visit(ir_texture *ir)
     */
    result_src = get_temp(ir->type);
    result_dst = st_dst_reg(result_src);
+   result_dst.writemask = (1 << ir->type->vector_elements) - 1;
 
    switch (ir->op) {
    case ir_tex: