drm/doc: Allow new UAPI to be used once it's in drm-next/drm-misc-next.

drm/doc: Allow new UAPI to be used once it's in drm-next/drm-misc-next.

I was trying to figure out if it was permissible to merge the Mesa
side of V3D's CSD support yet while it's in drm-misc-next but not
drm-next, and developers on #dri-devel IRC had differing opinions of
what the requirement was.

v2: Restrict to just drm-next or drm-misc-next on airlied's request.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190424220638.16222-1-eric@anholt.net
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>