Fix to print metrics for operations with no per-stage cycle counts

Operations like glTexImage* get a valid time from the timer query, but
get performance counter numbers of zero, (since the operation is
performed in a blit batch which cannot have performance-monitor
operations in it).

We had code in place to protect any divide-by-zero in this case, but
that case was mistakenly setting the resulting time to 0, so any
operations like this were not having their time reported.

To fix this, we can't compute any per-stage time, so we arbitrarily
use stage 0 as the place to store 100% of the time spent, but we
update this per-stage metric value to point to a NULL per-stage name
to avoid any lie in the report.