drm/udl: make control msg static const. (v2)

Thou shall not send control msg from the stack,
does that mean I can send it from the RO memory area?

and it looks like the answer is no, so here's
v2 which kmemdups.

Reported-by: poma
Tested-by: poma <poma@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>