drm/gma500: use NULL instead of using plain integer as pointer

This fixes the following sparse warnings:
drivers/gpu/drm/gma500/psb_drv.c:303:56: warning: Using plain integer as
NULL pointer

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1615969652-80225-1-git-send-email-yang.lee@linux.alibaba.com