drm/mediatek: Add spinlock for setting vblank event in atomic_begin

Add spinlock protection to avoid race condition on vblank event
between mtk_drm_crtc_atomic_begin() and mtk_drm_finish_page_flip().

Fixes: 119f5173 ("drm/mediatek: Add DRM Driver for Mediatek SoC MT8173.")
Signed-off-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
Suggested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Reviewed-by: Fei Shao <fshao@chromium.org>
Tested-by: Fei Shao <fshao@chromium.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20230920090658.31181-1-jason-jh.lin@mediatek.com/


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