drm/i915/wm: clear the plane ddb_y entries on plane disable

drm/i915/wm: clear the plane ddb_y entries on plane disable

The UV/Y plane DDB entriess are never cleared on
sk_wm_plane_disable_noatomic() and can leave stale DDB state
for NV12 planes on pre-Gen11 devices

Fixes: d34b59d5ba41 ("drm/i915: Add skl_wm_plane_disable_noatomic()")
Assisted-by: Copilot:claude-sonnet-4.6
Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patch.msgid.link/20260615203355.218578-2-vinod.govindapillai@intel.com
(cherry picked from commit 60f68a6ba298fd1e971a2d91576304bee89a16fc)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>