drm/i915/stolen: enforce the min_page_size contract

Since stolen can now be device local-memory underneath, we should try to
enforce any min_page_size restrictions when allocating pages.

Signed-off-by: CQ Tang <cq.tang@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421104658.304142-3-matthew.auld@intel.com