drm/amdgpu: fix unload driver issue for virtual display

Virtual display doesn't allocate amdgpu_encoder when initializing,
so will get invaild pointer if try to free amdgpu_encoder when
unloading driver.

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