drm/amdgpu/nbio7.11: fix IP version check

commit 8c1ecc71 upstream.

Use the helper function rather than reading it directly.

Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 2c8eeaaa)
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>