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

There is no need to cast fb_info.par to "struct atafb_par *", as the
former has type "void *".

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>