drivers/video: fsl-diu-fb: merge init_fbinfo() into install_fb()

Function init_fbinfo() is called only from install_fb(), and it's only a few
lines long.  Plus, it ignores the return code from fb_alloc_cmap().  Merge
its contents into install_fb() and handle errors properly.

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