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

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

[ Upstream commit 7995bd13296111d672d8c5959f5e81dbbbda5286 ]

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

Fixes: ac19f140bc27 ("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
Signed-off-by: Sasha Levin <sashal@kernel.org>