MLK-11765 video: mxsfb: change the fb buffer managment

The previous fb buffer management has two problems:

1. After reallocate a bigger buffer and free the old buffer,
   user space app doesn't know this and may continue accessing
   the old buffer.
2. The freed buffer contents will be lost.

So, this patch allocates a big enough fb buffer(32MB) from the
beginning and never reallocates it.

Signed-off-by: Fancy Fang <chen.fang@freescale.com>
(cherry picked from commit c76a37e342369675aa9ef2efde6373d288c2f013)

Conflicts:
	drivers/video/mxsfb.c