tests/kms_frontbuffer_tracking: fix fbc checking rules

Fix FBC checking rules for scaledprimary subtests

References: https://gitlab.freedesktop.org/drm/intel/issues/160


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