drm/via: Remove unecessary NULL check

The context_dtor callback is only called once we've successfully loaded
the driver, which means dev->dev_private is set up. The check is hence
pointless.

Also dev->dev_private is deref already above, so compilers are free
to elide it anyway.

Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>