drm/vgem: add missing platform_device_unregister() in vgem_init()

When vgem_init() get into out_put, the unregister call of
vgem_device->platform is missing. So add it before return.

Fixes: 363de9e7

 ("drm/vgem: Use drmm_add_final_kfree")
Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200810125942.186637-1-miaoqinglang@huawei.com