drm/bridge: split some definitions of ANX78xx to dedicated headers

Some definitions currently in analogix-anx78xx.h are not restricted to
the ANX78xx series, but also applicable to other DisplayPort
transmitters by Analogix.

Split out them to dedicated headers, and make analogix-anx78xx.h include
them.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Torsten Duwe <duwe@suse.de>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107135212.4D41E68BE1@verein.lst.de