drm: inline "struct drm_sigdata"

The sigdata structure is only used to group two fields in drm_device.
Inline it and make it an unnamed object.

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