drm/sun4i: tv: Remove unused mode_valid

The mode_valid implementation is pretty much a nop, let's remove it.

Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220728-rpi-analog-tv-properties-v2-34-459522d653a7@cerno.tech