drm/sun4i: tv: Merge mode_set into atomic_enable

Our mode_set implementation can be merged into our atomic_enable
implementation to simplify things, so let's do this.

Acked-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-v2-36-459522d653a7@cerno.tech