drm/amdgpu/virtual_dce: No need to pin the fb's bo

For virtual display, no need to pin the fb's bo.

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>