drm: remove unused and redundant callbacks

Drivers no longer have any need for these callbacks, and there are no
users. Zap. Zap-zap-zzzap-p-pp-p.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170713162538.22788-15-peda@axentia.se