drm/amdgpu/dce_virtual: remove error message for vega10

Vega10 also support virtual display, remove the error message.

Signed-off-by: Xiangliang.Yu <Xiangliang.Yu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>