drm/format-helper: Type fixes in format-helper tests

Change the source-buffer type of le32buf_to_cpu() to __le32* to
reflect endianness. Result buffers are converted to local endianness,
so instantiate them from regular u8 or u32 types.

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/20230102112927.26565-6-tzimmermann@suse.de