drm/mm: fix logic for selection of best fit block

This is from bug 25728.

[airlied: I'm just forwarding the patch for review, Thomas, ickle?]

Acked-by: Jerome Glisse <jglisse@redhat.com>
Acked-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>