drm/rockchip: fb: add missing header

This fixes the following sparse warnings:

drivers/gpu/drm/rockchip/rockchip_drm_fb.c:32:23: warning: symbol 'rockchip_fb_get_gem_obj' was not declared. Should it be static?
drivers/gpu/drm/rockchip/rockchip_drm_fb.c:315:24: warning: symbol 'rockchip_drm_framebuffer_init' was not declared. Should it be static?
drivers/gpu/drm/rockchip/rockchip_drm_fb.c:329:6: warning: symbol 'rockchip_drm_mode_config_init' was not declared. Should it be static?

Signed-off-by: John Keeping <john@metanate.com>