drm/ingenic: Support multiple panels/bridges

Support multiple panels or bridges connected to the same DPI output of
the SoC. This setup can be found for instance on the GCW Zero, where the
same DPI output interfaces the internal 320x240 TFT panel, and the ITE
IT6610 HDMI chip.

v2: No change

v3: Allow > 80-char lines where it makes sense

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716163846.174790-11-paul@crapouillou.net