drm/mediatek: fix kernel oops if no crtc is found

drm_crtc_from_index(0) might return NULL if there are no CRTCs
registered at all which will lead to a kernel oops in
mtk_drm_crtc_dma_dev_get(). Add the missing return value check.

Fixes: 0d9eee91 ("drm/mediatek: Add drm ovl_adaptor sub driver for MT8195")
Signed-off-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Eugen Hristev <eugen.hristev@collabora.com>
Reviewed-by: Eugen Hristev <eugen.hristev@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20230905084922.3908121-1-mwalle@kernel.org/Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

drivers/gpu/drm/mediatek/mtk_drm_drv.c

@@ -443,6 +443,7 @@ static int mtk_drm_kms_init(struct drm_device *drm)
 	struct mtk_drm_private *private = drm->dev_private;
 	struct mtk_drm_private *priv_n;
 	struct device *dma_dev = NULL;
+	struct drm_crtc *crtc;
 	int ret, i, j;
 
 	if (drm_firmware_drivers_only())
@@ -519,7 +520,9 @@ static int mtk_drm_kms_init(struct drm_device *drm)
 	}
 
 	/* Use OVL device for all DMA memory allocations */
-	dma_dev = mtk_drm_crtc_dma_dev_get(drm_crtc_from_index(drm, 0));
+	crtc = drm_crtc_from_index(drm, 0);
+	if (crtc)
+		dma_dev = mtk_drm_crtc_dma_dev_get(crtc);
 	if (!dma_dev) {
 		ret = -ENODEV;
 		dev_err(drm->dev, "Need at least one OVL device\n");