drm/tests: Fix endian warning

When compiling with sparse enabled, this warning is thrown:

  warning: incorrect type in argument 2 (different base types)
     expected restricted __le32 const [usertype] *buf
     got unsigned int [usertype] *[assigned] buf

Add a cast to fix it.

Fixes: 45311431 ("drm/format-helper: Add KUnit tests for drm_fb_xrgb8888_to_xrgb2101010()")
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250630090054.353246-1-jose.exposito89@gmail.com