drm/format-helper: Make "destination_pitch" test usable for mono

This test case uses an arbitrary pitch size, different of the default
one, to test if the conversions methods obey.

Change the "destination_pitch" colors to change the monochrome expected
result from being just zeros, as this makes the arbitrary pitch use
unusable.

Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230311125141.564801-3-arthurgrillo@riseup.net