drm/sun4i: Move the panel pointer from the TCON to the encoders

The TCON driver used to need the panel pointer in order to configure the
tcon according to the various parameters of the panel. However, this has
evolved over time (especially to support bridges), and therefore the panel
pointer isn't needed anymore by the TCON driver.

Move that pointer to the LVDS and RGB encoders drivers.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Tested-by: Vasily Khoruzhick <anarsoul@gmail.com> # tested on pinebook
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/13288b6b8f27b614a6c9aef348923c34b2803ad4.1551191081.git-series.maxime.ripard@bootlin.com