Selftest for drm damage helper iterator functions.

v2: Rebase to new test-drm_modeset.
Signed-off-by: NDeepak Rawat <drawat@vmware.com>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: NThomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: NThomas Hellstrom <thellstrom@vmware.com>