drm: vmwgfx: Remove unneeded CRTC .prepare() helper operation

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