drm/bridge: analogix_dp: add helpers for capture of frame CRCs

Add two simple functions that just take the drm_dp_aux from our struct
and calls the corresponding DP helpers with it.

v6: Pass to the DP helper the drm_crtc of the current connector (Sean Paul)

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170303133936.14964-4-tomeu.vizoso@collabora.com