drm/rockchip: cleanup unnecessary export symbol

Now rockchip_drm_vop.c is build into rockchipdrm.ko, so
no need to export following symbol anymore:
    rockchip_drm_dma_attach_device
    rockchip_drm_dma_detach_device
    rockchip_drm_dma_attach_device
    rockchip_drm_dma_detach_device
    rockchip_register_crtc_funcs
    rockchip_unregister_crtc_funcs
    rockchip_fb_get_gem_obj

Signed-off-by: Mark Yao <mark.yao@rock-chips.com>