drm/vgem: Fix return value check in vgem_init()

In case of error, the function platform_device_register_simple() returns
ERR_PTR() and never returns NULL. The NULL test in the return value
check should be replaced with IS_ERR().

Fixes: af33a919 ("drm/vgem: Enable dmabuf import interfaces")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Fix fixes: tag per Chris' review.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170521011939.8111-1-weiyj.lk@gmail.com