drm/bridge: sii902x: add input_bus_flags

The driver always sets InputBusFmt:EDGE to 0 (falling edge).

Add drm_bridge_timings's input_bus_flags to reflect that the bridge
samples on falling edges.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/deb02a1fd508c514472a4e660092f301e6681361.1558964241.git.jsarha@ti.com