drm: vkms: check status of alloc_ordered_workqueue

alloc_ordered_workqueue may fail and return NULL.
The fix returns ENOMEM when it fails to avoid potential NULL
pointer dereference.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190309043628.6078-1-kjlu@umn.edu