drm/amd: Interface change to support 64 bit page_table_base

amdgpu_gpuvm_get_process_page_dir should return the page table address
in the format expected by the pm4_map_process packet for all ASIC
generations.

Signed-off-by: Shaoyun Liu <Shaoyun.Liu@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>