drm: Add callbacks for late registering

Like what has been done for connectors add callbacks on encoder,
crtc and plane to let driver do actions after drm device registration.

Correspondingly, add callbacks called before unregister drm device.

version 2:
add drm_modeset_register_all() and drm_modeset_unregister_all()
to centralize all calls

version 3:
in error case unwind registers in drm_modeset_register_all
fix uninitialed return value
inverse order of unregistration in drm_modeset_unregister_all

version 4:
move function definitions in drm_crtc_internal.h
remove not needed documentation

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466519829-4000-1-git-send-email-benjamin.gaignard@linaro.org