Cut down on rescheduling in sceGeListEnqueue

Fixes ClaDun X2.  On the real PSP, it seems like this function should never reschedule (callback or not), although it might reschedule later.  ClaDun X2 breaks if it reschedules when it passes -1 (meaning, no signal callback.)

This is a bit of a hack but it should be a safe one.  Might possibly help other games.

-[Unknown]