drm/sun4i: tcon: Don't rely on encoders to enable the TCON

So far, we've required all the TCON-connected encoders to call the TCON
enable and disable functions.

This was made this way because in the RGB/LVDS case, the TCON is the CRTC
and the encoder. However, in all the other cases (HDMI, TV, DSI, etc.), we
have another encoder down the road that needs to be programmed.

We also needed to know which channel the encoder is connected to, which is
encoder-specific.

The CRTC's enable and disable callbacks can work just fine for our use
case, and we can get the channel to use just by looking at the type of
encoder, since that is fixed. Implement those callbacks, which will
remove some of the encoder boilerplate.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/90b4396e19b3eca61b2ebfdae0672074b88ad74d.1508231063.git-series.maxime.ripard@free-electrons.com