drm/amdgpu: report DC not supported if virtual display is enabled (v2)

Virtual display is non-atomic so report false to avoid checking
atomic state and other atomic things at runtime.

v2: squash into the sr-iov check

Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Acked-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>