drm/hdlcd: Clean up crtc hooks

Those are all no longer needed for a pure atomic driver.

Cc: Liviu Dudau <Liviu.Dudau@arm.com>
Tested-by: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464818821-5736-17-git-send-email-daniel.vetter@ffwll.ch