Not having checks for this isn't good.

I've checked igt and libdrm and they all already clear flags properly.
So we're lucky and should be able to sneak this ABI clarification in.

Testcase: igt/gem_wait/invalid-flags
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=85280Reviewed-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>