drm/amdgpu: Update pin_size values before unpinning BO

At least in theory, ttm_bo_validate may move the BO, in which case the
pin_size accounting would be inconsistent with when the BO was pinned.

Cc: stable@vger.kernel.org
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>