drm/i915: Zap call to drm_plane_helper_disable, v2.

The primary plane can still be configured when crtc is off,
furthermore this is also a noop now that affected planes are
added on modesets.

Changes since v1:
- Move commit so no frontbuffer_bits warnings are generated.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>