va: allocator: Decide the use derived flag when set_format().

The current allocator does not set use_derived flag to check the video info when set_format(). So it always creates a new image and updates the video info based on that. But When mapping the surface later, the image may derive from the surface. This causes a latent issue. On some platform, the image created by vaCreateImage has different pitch and stride from the image derived by vaDeriveImage. The surface is used by GPU HW and may have more limitation, such as size, stride and pitch. So when the image derived from that surface, its info is different from the new created one. We should decide the use_derived flag at allocator's init time.