drm/i915: Introduce and use intel_atomic_crtc_state_for_each_plane_state.

Instead of looking at drm_plane_state, look at intel_plane_state directly.

This will allow us to make the watermarks bigjoiner aware, when we make it
work for bigjoiner slave pipes as well.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004113514.17064-4-maarten.lankhorst@linux.intel.com


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>