drm: Update drm_device docs about embedding.

It's supported now! Spotted while reviewing Chris' patch to add a
release hook.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161208102847.3063-1-daniel.vetter@ffwll.ch
1 file changed