drm: radeon: fix sparse integer as NULL pointer warnings in radeon_mem.c

drivers/char/drm/radeon_mem.c:91:23: warning: Using plain integer as NULL pointer
drivers/char/drm/radeon_mem.c:116:28: warning: Using plain integer as NULL pointer
drivers/char/drm/radeon_mem.c:124:28: warning: Using plain integer as NULL pointer
drivers/char/drm/radeon_mem.c:177:26: warning: Using plain integer as NULL pointer
drivers/char/drm/radeon_mem.c:177:53: warning: Using plain integer as NULL pointer

Signed-off-by: Harvey Harrison <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

drivers/char/drm/radeon_mem.c

···
 
 	list_for_each(p, heap) {
 		int start = (p->start + mask) & ~mask;
-		if (p->file_priv == 0 && start + size <= p->start + p->size)
+		if (p->file_priv == NULL && start + size <= p->start + p->size)
 			return split_block(p, start, size, file_priv);
 	}
 
···
 	/* Assumes a single contiguous range.  Needs a special file_priv in
 	 * 'heap' to stop it being subsumed.
 	 */
-	if (p->next->file_priv == 0) {
+	if (p->next->file_priv == NULL) {
 		struct mem_block *q = p->next;
 		p->size += q->size;
 		p->next = q->next;
···
 		drm_free(q, sizeof(*q), DRM_MEM_BUFS);
 	}
 
-	if (p->prev->file_priv == 0) {
+	if (p->prev->file_priv == NULL) {
 		struct mem_block *q = p->prev;
 		q->size += p->size;
 		q->next = p->next;
···
 	 * 'heap' to stop it being subsumed.
 	 */
 	list_for_each(p, heap) {
-		while (p->file_priv == 0 && p->next->file_priv == 0) {
+		while (p->file_priv == NULL && p->next->file_priv == NULL) {
 			struct mem_block *q = p->next;
 			p->size += q->size;
 			p->next = q->next;