Enable MTRR on all VIA CPUs that claim support.

This may not be entirely correct either, but the existing check is
bogus.  I have both a C3 and a C7 that fail this check, but work fine.

MFC after:	2 weeks

sys/i386/i386/i686_mem.c

@@ -707,11 +707,8 @@ i686_mem_drvinit(void *unused)
 	switch (cpu_vendor_id) {
 	case CPU_VENDOR_INTEL:
 	case CPU_VENDOR_AMD:
-		break;
 	case CPU_VENDOR_CENTAUR:
-		if (cpu_exthigh >= 0x80000008)
-			break;
-		/* FALLTHROUGH */
+		break;
 	default:
 		return;
 	}