drivers/video: fsl-diu-fb: remove redundant default video mode

The framebuffer layer already uses the first video mode defined in the
fb_videomode array as a default, so there's no need to duplicate the
first entry into a stand-alone structure.

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