fbdev: hitfb: Use NULL for pointers

Replace integer constants with NULL. Resolves the following
warnings:

../drivers/video/fbdev/hitfb.c:447:23: warning: Using plain integer as NULL pointer
../drivers/video/fbdev/hitfb.c:465:23: warning: Using plain integer as NULL pointer

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Helge Deller <deller@gmx.de>