tests/kms_flip_event_leak: no need to clear black with black

created framebuffers were uselessly painted black which just wasted
time from ci.

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>