drm/atomic: Add missing () to function ref in kerneldoc

Pure drive-by while reading code&docs.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181012073441.21774-1-daniel.vetter@ffwll.ch