drm/bridge: cdns: Separate DSI and D-PHY configuration

The current configuration of the DSI bridge and its associated D-PHY is
intertwined. In order to ease the future conversion to the phy framework
for the D-PHY part, let's split the configuration in two.

Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0b3bea44e05745b65c23af7926ca546bc80a1bcc.1548085432.git-series.maxime.ripard@bootlin.com