drm/sun4i: Don't check for panel or bridge on TV TCONs

TV TCONs are always connected to TV or HDMI encoder, so it doesn't make
sense to check if panel or bridge is connected to them.

Check if TCON has channel 0 and only then check for connected panel or
bridges.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180625120304.7543-13-jernej.skrabec@siol.net