drm/radeon: Lower the severity of the radeon lockup messages.

abrt files a lot of bug reports when users get GPU lockups, but there's not really
enough context to do anything useful with them.  Given the lack of GPU context being
dumped, this patch removes the stack trace, so that abrt ignores the messages.

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>