drm/ttm: Don't move non-existing data

If ttm_bo_move_memcpy was instructed to move a non-populated ttm to
io memory, it would first populate the ttm, then move the data and then
destroy the ttm. That's stupid. However, some drivers might have relied on
this to clear io memory from old stuff. So instead of a NOP, which would
be the most efficient, just clear the destination.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>