drivers/video: fsl-diu-fb: remove broken screen blanking support

The function which is supposed to provide screen blanking support doesn't
actually do anything, so the framebuffer layer thinks the screen has
been blanked when it really isn't.  Remove the code completely for now.

A side-effect of this change is that the framebuffer console blanking now
works correctly.  Presumably this is because the console now receives -EINVAL
instead of '0' when it asks the driver to blank the screen, so the console
does it manually now.

A signficant refactoring of the driver is planned, and proper hardware
blanking support will added afterwards.

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