Just like we did for the TCON enable and disable, for historical reasons we
used to rely on the encoders calling the TCON mode_set function, while the
CRTC has a callback for that.

Let's implement it in order to reduce the boilerplate code.
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/faa3a4d511039af1d116270dfef3a8b60ca3591e.1508231063.git-series.maxime.ripard@free-electrons.com