drm/sun4i: hdmi: Move the mode_valid callback to the encoder

When attached to the connector, the mode_valid callback will only filter
the modes provided by the connector itself as part of its probe.

However, it will not be doing it when the mode is provided by the
userspace, which still might result in a broken configuration.

In order to enforce these constraints, move our mode_valid callback to the
encoder which doesn't have this behaviour.

Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
[maxime: Wrote the commit log in order to update the patch from the merged
	 v3 to the v4 that was correct.]
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0fa230a8-d01d-561a-f74f-6b4fd421255b@xs4all.nl