drm/amdgpu: Fix vram recover doesn't work after whole GPU reset (v2)

v1: Vmbo->shadow is used to back vram bo up when vram lost. So that we
should set shadow as vmbo->shadow to recover vmbo->bo
v2: Modify if(vmbo->shadow) shadow = vmbo->shadow as if(!vmbo->shadow)
continue;

Fixes: e18aaea7 ("drm/amdgpu: move shadow_list to amdgpu_bo_vm")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Lin.Cao <lincao12@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org