drm/sun4i: tcon: Reduce the scope of the LVDS error a bit

The current logic to deal with old DT missing the LVDS properties doesn't
take into account whether the LVDS output is supported in the first place,
resulting in spurious error messages on SoCs where it doesn't even matter.

Introduce a new TCON flag to list if LVDS is supported at all to prevent
this from happening.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180221125703.4595-1-maxime.ripard@bootlin.com