replace 0 with NULL for NULL pointers

This updates many places where 0 is used as NULL pointer.

There are a few warnings left when I build the default
configuration but they either relate to code
outside of mesa or where "None" is used instead.

Found with static analysis (smatch)

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12174>

src/glx/dri2_glx.c

@@ -710,7 +710,7 @@ static void show_fps(struct dri2_drawable *draw)
    struct timeval tv;
    uint64_t current_time;
 
-   gettimeofday(&tv, 0);
+   gettimeofday(&tv, NULL);
    current_time = (uint64_t)tv.tv_sec*1000000 + (uint64_t)tv.tv_usec;
 
    draw->frames++;