modeprint, modetest, proptest: cast __u64 to uint64_t

It seems that __u64 values are defined differently across systems. In
glibc it's defined as unsigned long, in Linux kernel headers
(int-ll64.h) as unsigned long long, and on FreeBSD as uint64_t so it
matches glibc. A temporal solution is to cast all __u64 values to
uint64_t to avoid warnings on Linux, but ideally we'd like a better fix
in the future.

See also: https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/212


for discussion.

Signed-off-by: Eleni Maria Stea <elene.mst@gmail.com>