anv/allocator: Add a sanity assertion in state stream finish.

anv/allocator: Add a sanity assertion in state stream finish.

We assert that the block offset we got while walking the list of blocks is
actually a multiple of the block size.  If something goes wrong and the GPU
decides to stomp on the surface state buffer we can end up getting
corruptions in our list of blocks.  This assertion makes such corruptions a
crash with a meaningful message rather than an infinite loop.