drm/format-helper: Fix test on big endian architectures

The tests fail on big endian architectures, like PowerPC:

 $ ./tools/testing/kunit/kunit.py run \
   --kunitconfig=drivers/gpu/drm/tests \
   --arch=powerpc --cross_compile=powerpc64-linux-gnu-

Transform the XRGB8888 buffer from little endian to the CPU endian
before calling the conversion function to avoid this error.

Fixes: 8f456104 ("drm/format-helper: Add KUnit tests for drm_fb_xrgb8888_to_rgb332()")
Reported-by: David Gow <davidgow@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220726230916.390575-2-jose.exposito89@gmail.com