drm: reference mode flags in DRM_CLIENT_CAP_* docs

drm: reference mode flags in DRM_CLIENT_CAP_* docs

In the docs for DRM_CLIENT_CAP_STEREO_3D and
DRM_CLIENT_CAP_ASPECT_RATIO, reference the DRM_MODE_FLAG_* defines
that get set when the cap is enabled.

Signed-off-by: Simon Ser <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Acked-by: Daniel Stone <[email protected]>
Acked-by: Pekka Paalanen <[email protected]>
Link: https://patchwork.freedesktop.org/patch/434201/