drm/radeon: Split off gart_get_page_entry ASIC hook from set_page_entry

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: Christian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>