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

drivers/gpu/drm/drm_crtc_helper.c

@@ -244,10 +244,6 @@ drm_crtc_prepare_encoders(struct drm_device *dev)
 		/* Disable unused encoders */
 		if (encoder->crtc == NULL)
 			drm_encoder_disable(encoder);
-		/* Disable encoders whose CRTC is about to change */
-		if (encoder_funcs->get_crtc &&
-		    encoder->crtc != (*encoder_funcs->get_crtc)(encoder))
-			drm_encoder_disable(encoder);
 	}
 }