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: 4531143 ("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

Author: JoseExposito

drivers/gpu/drm/tests/drm_format_helper_test.c

@@ -1033,7 +1033,7 @@ static void drm_test_fb_xrgb8888_to_xrgb2101010(struct kunit *test)
 		NULL : &result->dst_pitch;
 
 	drm_fb_xrgb8888_to_xrgb2101010(&dst, dst_pitch, &src, &fb, &params->clip, &fmtcnv_state);
-	buf = le32buf_to_cpu(test, buf, dst_size / sizeof(u32));
+	buf = le32buf_to_cpu(test, (__force const __le32 *)buf, dst_size / sizeof(u32));
 	KUNIT_EXPECT_MEMEQ(test, buf, result->expected, dst_size);
 
 	buf = dst.vaddr; /* restore original value of buf */