drm/dp: Move DisplayPort AUX bus helpers into dp/

Move drm_dp_aux_bus.c and its header file into the DP subdirectory
and update all users. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220114114535.29157-6-tzimmermann@suse.de