drivers/video: fsl-diu-fb: add default platform ops functions

The DIU driver requires some platform-specific functions to be defined,
but two them can be optional because most platforms implement them the
same way.  Functions set_gamma_table() and get_pixel_format() are only
needed because of quirks in the Freescale MPC8610 HPCD reference board.
For other boards, a generic implementation works, so we shouldn't
require the platform code to define them.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>