drm/bridge: mhdp8546: Add minimal format negotiation

With new connector model, mhdp bridge will not create the connector and
SoC driver will rely on format negotiation to setup the encoder format.

Support minimal format negotiations hooks in the drm_bridge_funcs.
Complete format negotiation can be added based on EDID data.
This patch adds the minimal required support to avoid failure
after moving to new connector model.

Signed-off-by: Nikhil Devshatwar <nikhil.nd@ti.com>
[a-bhatia1: Drop the output_fmt check condition]
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/20230606082142.23760-4-a-bhatia1@ti.com