drm/sun4i: tv: Get tcon and backend pointers from associated crtc

The drm_encoder structure provides us with a pointer to the crtc
currently tied to the encoder. Subsequently we can extract the
tcon and backend pointers from our crtc structure, instead of
getting it directly from the sun4i_drv structure.

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