The CRTC .prepare() helper operation is part of the legacy helpers and
is deprecated in favour of the .disable() helper operation. As the
vmwgfx driver provides a .disable() helper operation, and as the
.prepare() helper operation implementation is empty, we can remove it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170627211621.27767-5-laurent.pinchart+renesas@ideasonboard.com