drm/i915: Add reset count to error state

By default we keep only the error state from first hang. However
some sneaky user might have cleared the first error state and we
assume mistakenly that it is from first hang. As sometimes this
matters, it is better to explicitly store the reset count.

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>