intel_gtt: Use function to get the physical address

intel_gtt: Use function to get the physical address

The GTT PTEs that the tool is trying to compare is really about
addresses, and not the PTE itself. To accomplish this, make which
calculates the physical address we actually want.

This commit itself doesn't change any functionality; just the wording in
the code.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>