Revert "drm/amdkfd: return migration pages from copy function"

Revert "drm/amdkfd: return migration pages from copy function"

This reverts commit bd6093e2f1601c0c83906f5115a2efb6b93050b1.

migrate_vma_pages can fail if a CPU thread faults on the same page.
However, the page table is locked and only one of the new pages will
be inserted. The device driver will see that the MIGRATE_PFN_MIGRATE
bit is cleared if it loses the race.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Philip Yang <Philip.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>