drm/mediatek: Fix access violation in mtk_drm_crtc_dma_dev_get

Add error handling to check NULL input in
mtk_drm_crtc_dma_dev_get function.

While display path is not configured correctly, none of crtc is
established. So the caller of mtk_drm_crtc_dma_dev_get may pass
input parameter *crtc as NULL, Which may cause coredump when
we try to get the container of NULL pointer.

Fixes: cb1d6bcc ("drm/mediatek: Add dma dev get function")
Signed-off-by: Stuart Lee <stuart.lee@mediatek.com>
Cc: stable@vger.kernel.org
Reviewed-by: AngeloGioacchino DEl Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Macpaul Lin <macpaul.lin@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20231110012914.14884-2-stuart.lee@mediatek.com/Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

drivers/gpu/drm/mediatek/mtk_drm_crtc.c

@@ -926,7 +926,14 @@ static int mtk_drm_crtc_init_comp_planes(struct drm_device *drm_dev,
 
 struct device *mtk_drm_crtc_dma_dev_get(struct drm_crtc *crtc)
 {
-	struct mtk_drm_crtc *mtk_crtc = to_mtk_crtc(crtc);
+	struct mtk_drm_crtc *mtk_crtc = NULL;
+
+	if (!crtc)
+		return NULL;
+
+	mtk_crtc = to_mtk_crtc(crtc);
+	if (!mtk_crtc)
+		return NULL;
 
 	return mtk_crtc->dma_dev;
 }