drm/display: Split DisplayPort header into core and helper

Move DisplayPort protocol constants and structures into the new
header drm_dp.h, which can be used by DRM core components. The
existing header drm_dp_helper.h now only contains helper code for
graphics drivers. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220421073108.19226-5-tzimmermann@suse.de