i965: Move brw_vs_prog_data::outputs_written into VUE map.

Future patches will allow for there to be separate VUE maps when both
a geometry shader and a vertex shader are in use.  When this happens,
we will want to have correspondingly separate outputs_written
bitfields.  Moving outputs_written into the VUE map will make this
easy.

For consistency with the terminology used in the VUE map, the bitfield
is renamed to "slots_written" in the process.