drm/mm: kill color_search_free/get_block

drm/i915 is the only user of the color allocation handling and
switched to insert_node a while ago. So we can ditch this.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>