drm/tests: Fix drm_test_fb_xrgb8888_to_xrgb2101010() on big-endian

Fix failures on big-endian architectures on tests cases
single_pixel_source_buffer, single_pixel_clip_rectangle,
well_known_colors and destination_pitch.

Fixes: 15bda1f8 ("drm/tests: Add calls to drm_fb_blit() on supported format conversion tests")
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/20250630090054.353246-2-jose.exposito89@gmail.com