drm/format-helper: Normalize BT.601 factors to 256

BT.601 weights RGB components by certain factors to convert the
color to grayscale. Normalize the constants to 256 instead of 10.
Allows for slightly more precise rounding. The division by 256 can
be compiled as an 8-bit shift, which might be faster on some hardware.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250603161158.423962-1-tzimmermann@suse.de