drivers/video: fsl-diu-fb: remove unused ioctls

Remove some unused ioctl commands, and treat those commands as unsupported
instead of ignored.

Also remove struct mfb_alpha, which isn't used by any ioctl.  It may have
been once intended for MFB_SET_ALPHA, but that ioctl uses a different
data structure.

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