drm: Update docs after moving DisplayPort helpers around

DRM's DisplayPort helpers moved into the subdirectory dp/. Change
the documentation accordingly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: adb9d5a2 ("drm/dp: Move DisplayPort helpers into separate helper module")
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reported-by: Jani Nikula <jani.nikula@linux.intel.com>
Tested-by: Jani Nikula <jani.nikula@intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220120113647.22484-1-tzimmermann@suse.de