drm/amdgpu: Add vbios attribute only if supported

Not all devices carry VBIOS version information. Add the device
attribute only if supported.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Le Ma <le.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>