drm/bridge: analogix-anx78xx: Avoid drm_dp_link helpers

During the discussion of patches that enhance the drm_dp_link helpers it
was concluded that these helpers aren't very useful to begin with. Start
pushing the equivalent code into individual drivers to ultimately remove
them.

v4: use bulk DPCD writes if possible (Daniel Vetter)

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022145211.2258525-1-thierry.reding@gmail.com