v3dv,v3d: implement double-buffer mode

Double buffer mode splits the tile buffer size in half so we can start processing the next tile while the current one is being stored to memory. This mode is available only if MSAA is not enabled and can, in theory, improve performance by reducing tile store overhead, however, it comes at the cost of reducing the tile size, which also causes some overhead of its own.

Testing shows that this helps some cases (i.e the Vulkan Quake ports) but hurts others (i.e. Unreal Engine 4), so for the time being we don't enable this by default but we allow to enable it selectively by using V3D_DEBUG.