Remove side effects from VGA debug code

The VGA register dumping code was leaving ARX in data mode rather than index
mode, which could cause problems for later software accessing AR* registers.
Fix it to make sure it's in index mode when we're done.

Fixes #14434.

src/i830_debug.c

@@ -734,6 +734,7 @@ static void i830DumpAR(ScrnInfoPtr pScrn)
     }
     INREG8(st01);
     OUTREG8(0x3c0, orig_arx);
+    INREG8(st01); /* switch back to index mode */
 }
 
 void i830DumpRegs (ScrnInfoPtr pScrn)