drm/i915: Enable atomic on VLV/CHV

VLV/CHV watermarks are now able to handle the radiation, so
mark these platforms as ready for atomic.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Suggested-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170303151928.23053-5-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>