avcodec: Don't assume separate u and v planes in ff_alloc_picture

alloc_frame_buffer in ff_alloc_picture asserts that the linesize
of planes 1 and 2 are the same. If the pixfmt has a single uv
plane, like NV12, this won't be true.

So, let's only do this check if there are more than 2 planes.

We never hit this with previous hw formats because they don't set
linesize to meaningful values, but the cuda hw format sets the
values based on the underlying data layout.
