drm/format-helper: Update tests after BT.601 changes

drm/format-helper: Update tests after BT.601 changes

Commit a979a54165c2 ("drm/format-helper: Normalize BT.601 factors
to 256") improved rounding precision of the BT.601 calculation, which
impacts the results of soem of the format-helper tests. Adapt the test
to the new results.

v2:
- fix spelling in commit description

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: a979a54165c2 ("drm/format-helper: Normalize BT.601 factors to 256")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250613111711.136993-1-tzimmermann@suse.de