drm/amdgpu: fix unload driver issue for virtual display

drm/amdgpu: fix unload driver issue for virtual display

[ Upstream commit 3a1d19a29670aa7eb58576a31883d0aa9fb77549 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>