drm/bridge: sii902x: Set input_bus_flags in atomic_check

input_bus_flags are specified in drm_bridge_timings (legacy) as well
as drm_bridge_state->input_bus_cfg.flags

The flags from the timings will be deprecated. Bridges are supposed
to validate and set the bridge state flags from atomic_check.

Implement atomic_check hook for the same.

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/20230606082142.23760-7-a-bhatia1@ti.com