drm/i915/gtt: Add some range asserts

These should have been validated in the upper layers, but for sanity's
sake, repeat them.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190821155728.2839-2-chris@chris-wilson.co.uk