drm/i915: check i915_get_reset_stats_ioctl args

Insist that flags and pad fields are zero, so that
we can safely extend the interface in future.

Testcase: igt/gem_reset_stats/params

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>