drm/bridge: fix anx6345 compilation for v5.5

The anx6345 driver originally was copied from anx78xx.c, which has meanwhile
seen a few changes. In particular, the removal of drm_dp_link helpers and the
discontinuation to include drm_bridge.h from drm_crtc.h breaks compilation
in linux-5.5. Apply equivalents of these changes to anx6345.c.

Fixes: 6aa19269

 ("drm/bridge: Add Analogix anx6345 support")
Signed-off-by: Torsten Duwe <duwe@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20191112175940.GA13539@lst.de