appleramdac: Fix clock divisor 1x.

If bits 6 and 7 are 3, then the divisor should be 1 instead of 16.
0 -> 2x
1 -> 4x
2 -> 8x
3 -> 1x (not 16x).
This change affects RGB 12 inch dislay 512 x 384 60Hz mode in Platinum.

devices/video/appleramdac.cpp

@@ -168,7 +168,7 @@ void AppleRamdac::iodev_write(uint32_t address, uint16_t value) {
 }
 
 int AppleRamdac::get_clock_div() {
-    return 1 << ((dac_cr >> 6) + 1);
+    return 1 << (((dac_cr >> 6) + 1) & 3);
 }
 
 int AppleRamdac::get_pix_width() {