drm/amdgpu: add 64bit doorbell functions (v2)

Newer asics need 64 bit doorbells.

v2: fix comment (Nils)

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Ken Wang <Qingqing.Wang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>