drm/rockchip: make rockchip_gem_alloc_object static

The rockchip_gem_alloc_object function is not exported so
make it static to avoid the following sparse warning:

drivers/gpu/drm/rockchip/rockchip_drm_gem.c:297:28: warning: symbol 'rockchip_gem_alloc_object' 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/20191009121022.17478-1-ben.dooks@codethink.co.uk