add comment explaining requirement

src/video_core/pica/pica_core.cpp

@@ -58,6 +58,8 @@ void PicaCore::InitializeRegs() {
     // Values initialized by GSP
     regs.internal.irq_autostop = 1;
     regs.internal.irq_mask = 0xFFFFFFF0;
+    // Older versions of libctru didn't initialize this, initialize it here to avoid endless black screen
+    // not needed on actual hardware due to previous software already having set it up
     regs.internal.irq_compare = 0x12345678;
 
     auto& framebuffer_top = regs.framebuffer_config[0];