This helper is supposed to validate or reject the modeline before it
applied by the mode setting. Currently this function has been dropped,
it was previously set to a dummy function that always returned true. For
both cases, this means that userspace can ask for a bad modeline that
will be always accepted.

On some platforms, like Rockchip, the drm dw_hdmi-rockchip variant driver
already implements the atomic_check drm helper, so mode_fixup cannot be
handled and implemented there (as drm_atomic_helper relies on either
atomic_check or mode_fixup).

This commit implements this helper. It only checks that this mode is
correct from the connector point of view.
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Romain Perier <romain.perier@collabora.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170407121743.4142-1-romain.perier@collabora.com