drm/ttm: consistently use reservation_object_unlock

Instead of having a confusing wrapper or call the underlying ww_mutex
function directly.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-and-Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>