drm/rockchip: lvds: avoid duplicating drm_bridge_attach

drm_bridge_attach takes care of these assignments, so there is no need
to open-code them a second time.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180502074025.12421-3-peda@axentia.se