drm/amdgpu/nbio7.7: fix IP version check

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 22b9555b)
Cc: stable@vger.kernel.org