drm/file: Don't set master on in-kernel clients

It only makes sense for userspace clients.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180618141739.48151-3-noralf@tronnes.org