tests/kms_plane: add source clamping test

Add test which create fb bigger than plane and clamp fb
from all sides.

v2 (Maarten Lankhorst): Fix typo, create different color
border around clamped fb where during test border should
stay outside visible area.

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>