drm: arcpgu: Remove CRTC .prepare() helper operation

The CRTC helper .prepare() operation is legacy code, the atomic helpers
prefer the .disable() operation. As the arcpgu driver implements the
.disable() and .prepare() operations identicallly, .prepare() can be
removed.

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/20170627211621.27767-2-laurent.pinchart+renesas@ideasonboard.com