drm/mediatek: mtk_drm_crtc: Add checks for devm_kcalloc

As the devm_kcalloc may return NULL, the return value needs to be checked
to avoid NULL poineter dereference.

Fixes: 31c5558d ("drm/mediatek: Refactor plane init")
Signed-off-by: ruanjinjie <ruanjinjie@huawei.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20221205095115.2905090-1-ruanjinjie@huawei.com/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>