get_page_entry calculates the GART page table entry, which is just written
to the GART page table by set_page_entry.

This is a prerequisite for the following fix.
Reviewed-by: NChristian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: NMichel Dänzer <michel.daenzer@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>