drm: kselftest for drm_mm and color eviction

Check that after applying the driver's color adjustment, eviction
scanning finds a suitable hole.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161222083641.2691-19-chris@chris-wilson.co.uk