h264: move the DPB init/uninit to init_context()/free_context()

Currently, the DPB is initialized in alloc_tables() and uninitialized in
free_tables(), but those functions manage frame size-dependent
variables, so DPB management does not logically belong in there.

Since we want the init/uninit to happen exactly once per the context
lifetime, init_context()/free_context() are the proper place for this
code.
