drm/amdgpu: fix shadow BO restoring

Don't grab the reservation lock any more and simplify the handling quite
a bit.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>