drm/sun4i: Set TCON clock inside sun4i_tconX_mode_set

Currently we are configuring the TCON's dot clock or special clock
directly from the encoder mode_set functions. Since we already
provide mode_set helper functions for the TCON's 2 channels, we
can set the respective clock from those helpers, and reduce the
exposure of the TCON's internals.

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