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>

drivers/gpu/drm/rockchip/rockchip_drm_drv.c

@@ -55,14 +55,12 @@ int rockchip_drm_dma_attach_device(struct drm_device *drm_dev,
 
 	return arm_iommu_attach_device(dev, mapping);
 }
-EXPORT_SYMBOL_GPL(rockchip_drm_dma_attach_device);
 
 void rockchip_drm_dma_detach_device(struct drm_device *drm_dev,
 				    struct device *dev)
 {
 	arm_iommu_detach_device(dev);
 }
-EXPORT_SYMBOL_GPL(rockchip_drm_dma_detach_device);
 
 int rockchip_register_crtc_funcs(struct drm_crtc *crtc,
 				 const struct rockchip_crtc_funcs *crtc_funcs)
@@ -77,7 +75,6 @@ int rockchip_register_crtc_funcs(struct drm_crtc *crtc,
 
 	return 0;
 }
-EXPORT_SYMBOL_GPL(rockchip_register_crtc_funcs);
 
 void rockchip_unregister_crtc_funcs(struct drm_crtc *crtc)
 {
@@ -89,7 +86,6 @@ void rockchip_unregister_crtc_funcs(struct drm_crtc *crtc)
 
 	priv->crtc_funcs[pipe] = NULL;
 }
-EXPORT_SYMBOL_GPL(rockchip_unregister_crtc_funcs);
 
 static struct drm_crtc *rockchip_crtc_from_pipe(struct drm_device *drm,
 						int pipe)

drivers/gpu/drm/rockchip/rockchip_drm_fb.c

@@ -39,7 +39,6 @@ struct drm_gem_object *rockchip_fb_get_gem_obj(struct drm_framebuffer *fb,
 
 	return rk_fb->obj[plane];
 }
-EXPORT_SYMBOL_GPL(rockchip_fb_get_gem_obj);
 
 static void rockchip_drm_fb_destroy(struct drm_framebuffer *fb)
 {