drm/vkms: provide a parent device to drm_dev_init()

Earlier commit updated the vgem driver to improve the topology, by
passing a platform device as parent to drm_dev_init(). Shortly
afterwords we updated the core function to BUG() in order to catch any
buggy drivers passing NULL as parent.

While I missed the vkms driver (as the patch predates vkms by a few
months), the BUG caught the issue within couple of hours.

Swap the drm_dev_init <> platform_device_register_simple order, to
the driver back to life.

Fixes: f08877e7 ("drm: BUG_ON if passing NULL parent to drm_dev_init")
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Deepak Sharma <deepak.sharma@amd.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181026100550.625-1-emil.l.velikov@gmail.com