drm/bridge: sii902x: Support format negotiation hooks

With new connector model, sii902x will not create the connector, when
DRM_BRIDGE_ATTACH_NO_CONNECTOR is set and SoC driver will rely on format
negotiation to setup the encoder format.

Support format negotiations hooks in the drm_bridge_funcs.
Use helper functions for state management.

Input format is selected to MEDIA_BUS_FMT_RGB888_1X24 as default, as is
the case with older model.

Signed-off-by: Aradhya Bhatia <a-bhatia1@ti.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230606082142.23760-6-a-bhatia1@ti.com