video: fbdev: fsl-diu-fb: Remove VLA usage

In the quest to remove all stack VLA usage from the kernel[1], this moves
the buffer off the stack (since it could be as much as 1024 bytes), and
uses a new area in the cursor data structure. Additionally adds missed
documentation and removes redundant assignments.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com



Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Timur Tabi <timur@kernel.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>