drm/selftests: Select DRM_DP_HELPER

Resolve warnings about non-existing symbols by selecting DRM_DP_HELPER.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: adb9d5a2 ("drm/dp: Move DisplayPort helpers into separate helper module")
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reported-by: kernel test robot <lkp@intel.com>
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/20220118154418.25932-1-tzimmermann@suse.de