tests/kms_plane: fix pixel format (-clamping) tests

cdclk caused modesets mid test will mess up crc sequence
where two consecutive frames will get same crc. If there come
modeset restart round for given set as modeset will anyway
happen only on first color.

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>