drm: document minimum kernel version for DRM_CLIENT_CAP_*

The kernel versions including the following commits are referenced:

DRM_CLIENT_CAP_STEREO_3D
61d8e328 ("drm: Add a STEREO_3D capability to the SET_CLIENT_CAP ioctl")

DRM_CLIENT_CAP_UNIVERSAL_PLANES
681e7ec7 ("drm: Allow userspace to ask for universal plane list (v2)")
c7dbc6c9 ("drm: Remove command line guard for universal planes")

DRM_CLIENT_CAP_ATOMIC
88a48e29 ("drm: add atomic properties")
8b72ce15 ("drm: Always enable atomic API")

DRM_CLIENT_CAP_ASPECT_RATIO
7595bda2 ("drm: Add DRM client cap for aspect-ratio")

DRM_CLIENT_CAP_WRITEBACK_CONNECTORS
d67b6a20

 ("drm: writeback: Add client capability for exposing writeback connectors")

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Daniel Stone <daniels@collabora.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/434202/