drm/amdgpu: enable virtual display feature for navi10

Virtual display is a pure sw implementation of KMS for use
in virtualization and for bring up and emulation.

Signed-off-by: Le.Ma <Le.Ma@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>