drm/sun4i: DW HDMI: Expand algorithm for possible crtcs

drm_of_find_possible_crtcs() doesn't work when DW HDMI encoder is
connected to TCON (crtc) through mux in TCON TOP.

In that case TCON TOP HDMI mux input port has to be manually traversed
and checked if it matches any known crtc.

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/20180625120304.7543-23-jernej.skrabec@siol.net