drm/i915/gen11: Link nv12 Y and UV planes in the atomic state, v5.

To make NV12 working on icl, we need to update 2 planes simultaneously.
I've chosen to do this in the CRTC step after plane validation is done,
so we know what planes are (in)visible. The linked Y plane will get
updated in intel_plane_update_planes_on_crtc(), by the call to
update_slave, which gets the master's plane_state as argument.

The link requires both planes for atomic_update to work,
so make sure skl_ddb_add_affected_planes() adds both states.

Changes since v1:
- Introduce icl_is_nv12_y_plane(), instead of hardcoding sprite numbers.
- Put all the state updating login in intel_plane_atomic_check_with_state().
- Clean up changes in intel_plane_atomic_check().
Changes since v2:
- Fix intel_atomic_get_old_plane_state() to actually return old state.
- Move visibility changes to preparation patch.
- Only try to find a Y plane on gen11, earlier platforms only require
  a single plane.
Changes since v3:
- Fix checkpatch warning about to_intel_crtc() usage.
- Add affected planes from icl_add_linked_planes() before check_planes(),
  it's a cleaner way to do this. (Ville)
Changes since v4:
- Clear plane links in icl_check_nv12_planes() for clarity.
- Only pass crtc_state to icl_check_nv12_planes().
- Use for_each_new_intel_plane_in_state() in icl_check_nv12_planes.
- Rename aux to linked. (Ville)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181022135152.15324-1-maarten.lankhorst@linux.intel.com
[mlankhorst: Change bool slave to u32, to satisfy checkpatch]
[mlankhorst: Add WARN_ON's based on Ville's suggestion]