drm/dp: switch drm_dp_downstream_*() helpers to struct drm_edid

Prefer struct drm_edid where possible. With limited users for the
drm_dp_downstream_*() helpers, this is fairly straightforward.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231004162149.2802113-1-jani.nikula@intel.com