drm/amdgpu: Add only valid firmware version nodes

Show only firmware version attributes that have valid version. Hide
others.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>