Simplify CRTC enable.

v2: Don't forget DSI (Daniel)
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1ad8f3ccaed99a5f03e3d53345221a25ad0be50f.1507200657.git.jani.nikula@intel.com