video/powerpc: don't touch DIU registers that we don't need

Several DIU registers were being initialized either unnecessarily or to
wrong values.

1) All interrupts were enabled even though there's no interrupt handler.
   Interrupts were left enabled when booting Linux.

2) Don't configure a dummy area descriptor, since we don't support ADs
   in U-Boot.

3) Don't configure any write-back buffer registers, since we don't use
   that mode.

4) The default values for the THRESHOLDS, SYN_POL, and PLUT registers
   should be used, so don't touch those registers either.

Signed-off-by: Timur Tabi <timur@freescale.com>