drm/amdgpu/display: drop unused variable

Remove unused variable.

Fixes: e7d9560aea ("Revert "drm/amd/display: Fix overlay validation by considering cursors"")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -10120,7 +10120,7 @@ static int validate_overlay(struct drm_atomic_state *state)
 {
 	int i;
 	struct drm_plane *plane;
-	struct drm_plane_state *old_plane_state, *new_plane_state;
+	struct drm_plane_state *new_plane_state;
 	struct drm_plane_state *primary_state, *overlay_state = NULL;
 
 	/* Check if primary plane is contained inside overlay */