The buffer map/unmap code was in an early version of this before it
was committed, but the unmap was never removed. While wrong, this
was harmless (and therefore unnoticed) because the buffers can't be
mapped at this point - all drivers just did nothing with the call.
When decoding interlaced pictures, the structure is reused to render
to the same surface twice. The parameter buffers were not being
cleared, which caused the i965 driver to error out.
