drm/dp: Move DP declarations into separate header file

Split the DP declarations from other helpers before moving the
DP functions into a separate module.

v2:
	* forward-declare struct drm_dp_aux (Jani)
	* add include guards in drm_dp_helper_internal.h

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220114114535.29157-3-tzimmermann@suse.de