intel: Transition intel_region_map() to being a miptree operation.

intel: Transition intel_region_map() to being a miptree operation.

I'm trying to move us away from the region structure, and all the
callers are currently dereferencing a miptree to get the region.

In this change, the map_refcount is dropped.  However, the bo->virtual is
itself map refcounted, so that's already dealt with.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>