drm/panfrost: Make panfrost_gem_free_object() static

Fix sparse warning:

drivers/gpu/drm/panfrost/panfrost_gem.c:17:6:
 warning: symbol 'panfrost_gem_free_object' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190416150051.34092-1-yuehaibing@huawei.com