drm/format-helper: Fix test-input format conversion

Convert test input for format helpers from host byte order to
little-endian order. The current code does it the other way around,
but there's no effective difference to the result.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230102112927.26565-4-tzimmermann@suse.de