vga: change PLL input to 2MHz.
vga: change PLL input to 2MHz.

The STM32F4xx reference manual makes an off-hand comment that 2MHz
input may yield lower jitter.  In basic tests this appears to be
true.

cbiffle/m4vgalib,cbiffle/m4vgalib,cbiffle/m4vgalib,cbiffle/m4vgalib