drm/i915: Correctly reject invalid flags for wait_ioctl

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: Daniel Vetter <daniel.vetter@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=85280
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>