xvmc: force assertion in XvMC tests

xvmc: force assertion in XvMC tests

This follows the src/util/u_atomic_test.c model of undefining NDEBUG
unconditionally throughouth the XvMC tests, to force asserts regardless
of debug mode.

The comment on u_atomic_test.c is also fixed (read 'debug' where it
should have been 'release').

v2: s/debug/release/ in relevant comments

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
[Emil Velikov: keep the src/util/ hunk as separate patch]
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>