drm/amdgpu: free vkms_output after use

Signed-off-by: Flora Cui <flora.cui@amd.com>
Reviewed-by: Leslie Shi <Yuliang.Shi@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>