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>