drm/amdgpu: fix unload driver fail

dc_destroy should be called amdgpu_cgs_destroy_device,
as it will use cgs context to read or write registers.

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