drm: Add missing __user annotation

The drm_copy_field() function copies strings into userspace buffers, so
the first parameter needs to have a __user annotation to avoid warnings
from the sparse checker.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>