i965g: make the winsys responsible for all buffer->offset handling

i965g: make the winsys responsible for all buffer->offset handling

The winsys now inserts the presumed offset into referring buffers from
inside of bo_emit_reloc().  Remove the many locally coded places where
this was happening in the driver and eliminate the worry of getting it
wrong.

No longer need to expose offset values to the driver at all, so no need
to worry about what to do in the driver when they change.  Just use
zero values wherever we had offsets previously -- the relocations will
fix it all up for us.