drm/sun4i: hdmi: add support for ddc-i2c-bus property

Sometimes it is desirabled to use a separate i2c controller for ddc
access.  This adds support for the ddc-i2c-bus property of the
hdmi-connector node, using the specified controller if provided.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190328130249.19356-1-mans@mansr.com