drivers/video: fsl-diu-fb: merge fsl_diu_alloc() into map_video_memory()

Functions fsl_diu_alloc() and fsl_diu_free() were only being called by
map_video_memory() and unmap_video_memory(), respectively.

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