drm/i915: Tweak plane ddb allocation tracking

Let's store the plane allocation in a manner which more closely
matches how the hw operates. That is, we store the packed/CbCr
ddb in one struct, and the Y ddb in another. Currently we're
storing packed/Y in one struct, CbCr in the other.

This also works pretty well for icl+ where the UV plane is
the main plane and the Y plane is subservient to it. Although
in this case we do not even use ddb_y as we do the ddb allocation
in terms of hw planes.

v2: Rebase

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220303191207.27931-2-ville.syrjala@linux.intel.com