drm/dp: Move public DisplayPort headers into dp/

Move all public DisplayPort headers into dp/ and update users. No
functional changes.

v3:
	* rebased onto latest drm-tip

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220114114535.29157-5-tzimmermann@suse.de