drm/amdgpu: replace readq/writeq with atomic64 operations

what we really want is a read or write that is guaranteed to be 64 bits
at a time, atomic64 operations are supported on all architectures

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>