drm/i915: make gpu hangman more resilient

- reset the stop_rings infrastructure while resetting the hw to
  avoid angering the hangcheck right away (and potentially declaring
  the gpu permanently wedged).

- ignore reset failures when hanging due to the hangman - we don't
  have reset code for all generations.

v2: Ensure that we only ignore reset failures when the hw reset is not
implemented and not when it failed.

Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>