drm: Memory fragmentation from lost alignment blocks

If the block needs an alignment but otherwise fits exactly into the tail,
then the split-off block from the start would remain marked as non-free.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>