drm/amdgpu/vcn: add shared memory restore after wake up from sleep.

VCN shared memory needs restore after wake up during S3 test.

v2: Allocate shared memory saved_bo at sw_init and free it in sw_fini.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>