drm/mediatek: Add display merge start/stop API for cmdq support

Add merge start/stop API for cmdq support. The ovl_adaptor merges
are configured with each drm plane update. Need to enable/disable
merge with cmdq making sure all the settings taken effect in the
same vblank.

Signed-off-by: Nancy.Lin <nancy.lin@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Bo-Chen Chen <rex-bc.chen@mediatek.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20220620091930.27797-6-nancy.lin@mediatek.com/


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