video: fbdev: atari: Remove unneeded casts to void *

Arbitrary pointers can be passed to functions accepting "void *" without
casting.

Remove the casts, as they make it impossible to validate types.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>