drm/sun4i: tv: Switch to drm_of_find_possible_crtcs

Now that the crtcs have their .port field set properly, we can use
drm_of_find_possible_crtcs to find the connected crtcs, instead of
hardcoding the first crtc as usable. The new code also defers binding
when the upstream crtc hasn't been registered yet.

This makes it easier to support multiple tcons/crtcs.

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