drm/vkms: add missing platform_device_unregister() in vkms_init()

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

Fixes: ac19f140 ("drm/vkms: 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/20200810130011.187691-1-miaoqinglang@huawei.com