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

Documentation/gpu/drm-kms-helpers.rst

@@ -235,6 +235,9 @@ Display Port Helper Functions Reference
 .. kernel-doc:: drivers/gpu/drm/display/drm_dp_helper.c
    :doc: dp helpers
 
+.. kernel-doc:: include/drm/display/drm_dp.h
+   :internal:
+
 .. kernel-doc:: include/drm/display/drm_dp_helper.h
    :internal: