drm/i915/stolen: treat stolen local as normal local memory

Underneath it's the same stuff, so things like the PTE_LM bits for the
GTT should just keep working as-is.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421104658.304142-2-matthew.auld@intel.com