drivers/video: fsl-diu-fb: add support for set_gamma ioctls

The MPC5121 BSP comes with a gamma_set utility that initializes the gamma
table via an ioctl.  Unfortunately, the ioctl number that utility uses
is defined improperly, but we can still support it.

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