drm/i915: WARN if the bios reserved range is bigger than stolen size

v2: Bail out if we hit the WARN_ON to avoid fallout later on. Spotted
by Chris Wilson.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>