drm: Avoid NULL dereference of drm_device.dev

For a virtual device, drm_device.dev is NULL, so becareful not to
dereference it unconditionally in core code such as drm_dev_register().

Fixes: 75f6dfe3 ("drm: Deduplicate driver initialization message")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161230141639.10487-1-chris@chris-wilson.co.uk