drm/mediatek: Fix can't get component for external display plane.

The original logic is ok for primary display, but will not find out
component for external display.

For example, plane->index is 6 for external display, but there are only
2 layer nr in external display, and this condition will never happen:
if (plane->index < (count + mtk_ddp_comp_layer_nr(comp)))

Fix this by using the offset of the plane to mtk_crtc->planes as index,
instead of plane->index.

Fixes: d6b53f68 ("drm/mediatek: Add helper to get component for a plane")
Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>

drivers/gpu/drm/mediatek/mtk_drm_crtc.c

@@ -215,11 +215,12 @@ struct mtk_ddp_comp *mtk_drm_ddp_comp_for_plane(struct drm_crtc *crtc,
 	struct mtk_drm_crtc *mtk_crtc = to_mtk_crtc(crtc);
 	struct mtk_ddp_comp *comp;
 	int i, count = 0;
+	unsigned int local_index = plane - mtk_crtc->planes;
 
 	for (i = 0; i < mtk_crtc->ddp_comp_nr; i++) {
 		comp = mtk_crtc->ddp_comp[i];
-		if (plane->index < (count + mtk_ddp_comp_layer_nr(comp))) {
-			*local_layer = plane->index - count;
+		if (local_index < (count + mtk_ddp_comp_layer_nr(comp))) {
+			*local_layer = local_index - count;
 			return comp;
 		}
 		count += mtk_ddp_comp_layer_nr(comp);