Documentation: vkms: clarify devres managed reference cleanup

added documentation to drm_dev_unregister clarifying that devres managed
devices allocated with devm_drm_dev_alloc do not require calls to
drm_dev_put.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Brandon Pollack <brpol@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230425080240.3582324-1-brpol@chromium.org