vc4: Consume the implicit varyings for points and lines.

vc4: Consume the implicit varyings for points and lines.

We were triggering simulator assertion failures for not consuming these,
and presumably we want to actually make use of them some day (for things
like point/line antialiasing)

Note that this has the qreg index as 0, which is the same index as the
first GL varyings read.  This doesn't matter currently, since that number
isn't used for anything except dumping.