drm/i915/wm: Verify the correct plane DDB entry

drm/i915/wm: Verify the correct plane DDB entry

Actually verify the DDB entry for the plane we're looking
at instead of always verifying the cursor DDB.

Fixes: 7d4561722c3b ("drm/i915: Tweak plane ddb allocation tracking")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260324134843.2364-5-ville.syrjala@linux.intel.com
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
(cherry picked from commit f002f7c7439de18117a31ca84dc87a59719c3dd6)
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>