video: fbdev: amifb: add FIXMEs about {put,get}_user() failures

Since we lack the hardware (or proper emulator setup) for
testing needed changes add FIXMEs to document the issues
(so at least they are not forgotten).

Cc: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/380c0494-ed02-b2be-65b0-d385627fb894@samsung.com