drm/i915: Don't WARN about ring idle bit on gen2

Gen2 doesn't have the ring idle/stop bits in the SCPD/MI_MODE register,
so don't go spewing warnings about the state of those bits.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>