drm/vmwgfx: Use correct vmw_legacy_display_unit pointer

The "entry" pointer is an offset from the list head and it doesn't
point to a valid vmw_legacy_display_unit struct.  Presumably the
intent was to point to the last entry.

Also the "i++" wasn't used so I have removed that as well.

Fixes: d7e1958d ("drm/vmwgfx: Support older hardware.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Roland Scheidegger <sroland@vmware.com>