drm/sun4i: dw-hdmi: Make mode_valid function configurable

Since it is not possible to access sun8i-dw-hdmi driver private data
inside mode_valid function, make it configurable. That way different
versions of HDMI controllers can set different function, depending on
it's limitations.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181104182705.18047-17-jernej.skrabec@siol.net