drm: dw-hdmi: Implement the mode_fixup drm helper

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