drm/amdgpu: fix the memory corruption on S3

psp->cmd will be used on resume phase, so we can not free it on hw_init.
Otherwise, a memory corruption will be triggered.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Tested-by: Xiaojie Yuan <Xiaojie.Yuan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org