drm/crtc-helpers: WARN when used with atomic drivers

Motivated by an oversight of mine when looking at the atomic bochs
conversion. For consistency also switch over to the same style as used
elsewhere (e.g. in drm_mode_set_config_internal).

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190110103045.26821-1-daniel.vetter@ffwll.ch