drm/sun4i: rgb: Pass tcon pointer when initializing RGB encoder

The RGB encoder represents channel 0 of the TCON. Instead of fetching
the pointer to its TCON from the main sun4i_drv structure, pass it in
as part of the init call, save it, and use it directly in the encoder
and connector callbacks.

We can also drop the otherwise unused sun4i_drv pointer.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>