A simple assert to ensure that we don't overflow start + size when
initialising the drm_mm, or its scanner.

In future, we may want to switch to tracking the value of ranges (rather
than size) so that we can cover the full u64, for example like resource
tracking.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NJoonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161222083641.2691-26-chris@chris-wilson.co.uk