drm/amdgpu: Virtual display need to support multiple ctrcs

The crtc num is determined by virtual_display parameter.

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Monk Liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>