drm: drop unused drm_crtc callback

struct drm_encoder_helper_funcs included a callback
named drm_crtc.

There are no users left - so drop it.
There was one reference in drm_crtc_helper.c,
which checked if the value was not NULL.
As it was never assigned this check could be dropped.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200215173342.GA7458@ravnborg.org