Include rockchip_drm_drv.h for definition of vop_platform_driver
to avoid the following sparse warning:

drivers/gpu/drm/rockchip/rockchip_vop_reg.c:982:24: warning: symbol 'vop_platform_driver' was not declared. Should it be static?
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20191009132134.18384-1-ben.dooks@codethink.co.uk