drm/ttm: fix the type mismatch error on sparc64

drm/ttm: fix the type mismatch error on sparc64

__fls() on sparc64 return "int", but here it is expected as "unsigned
long" (x86). It will cause the build errors because the warning becomes
fatal while it is using sparc configuration. As suggested by Linus, it
can use min_t instead of min to force the type as "unsigned int".

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210907100302.3684453-1-ray.huang@amd.com