R600/SI: Fix config value for number of gprs

In r217636, the value stored in KernelInfo.Num[VS]GPRSs was changed from
the highest GPR index used to the number of gprs in order to be
consistent with the name of the variable.

The code writing the config values still assumed that the value in this
variable was the highest GPR index used, which caused the compiler to
over report the number of GPRs being used.

https://bugs.freedesktop.org/show_bug.cgi?id=84089