drm/sun4i: Use embedded tcon pointer to get the tcon's output port node

A pointer to the underlying tcon of the crtc was added to the sun4i_crtc
structure in "drm/sun4i: Add backend and tcon pointers to sun4i_crtc".
However the crtc init function was still using the copy from sun4i_drv
to set drm_crtc.port. This was an oversight when the patches were
reordered.

Switch to using the embedded tcon pointer to get the tcon's ouptut port
and assign it to drm_crtc.port.

This makes it possible to remove the usage of sun4i_drv completely in
subsequent patches.

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