drivers/video: fsl-diu-fb: don't touch registers for unused features

We don't use the writeback buffer, so don't initialize the registers for
that feature.

The default value for SYN_POL is already zero, so don't re-initialize it.

Writing the INT_STATUS register does nothing.

The value that we write to the PLUT register only makes sense on the
MPC8610 and P1022, so don't touch that register on the MPC5121.

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