drm/mediatek: Add cnt checking for coverity issue

CERT-C Characters and Strings (CERT STR31-C)
all_drm_priv[cnt] evaluates to an address that could be at negative
offset of an array.

In mtk_drm_get_all_drm_priv():
Guarantee that storage for strings has sufficient space for character
data and the null terminator.

So change cnt to unsigned int and check its max value.

Fixes: 1ef7ed48 ("drm/mediatek: Modify mediatek-drm for mt8195 multi mmsys support")
Signed-off-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20230714094908.13087-3-jason-jh.lin@mediatek.com/Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

drivers/gpu/drm/mediatek/mtk_drm_drv.c

@@ -354,7 +354,7 @@ static bool mtk_drm_get_all_drm_priv(struct device *dev)
 	const struct of_device_id *of_id;
 	struct device_node *node;
 	struct device *drm_dev;
-	int cnt = 0;
+	unsigned int cnt = 0;
 	int i, j;
 
 	for_each_child_of_node(phandle->parent, node) {
@@ -375,6 +375,9 @@ static bool mtk_drm_get_all_drm_priv(struct device *dev)
 		all_drm_priv[cnt] = dev_get_drvdata(drm_dev);
 		if (all_drm_priv[cnt] && all_drm_priv[cnt]->mtk_drm_bound)
 			cnt++;
+
+		if (cnt == MAX_CRTC)
+			break;
 	}
 
 	if (drm_priv->data->mmsys_dev_num == cnt) {