drm/radeon: NULL noise: drivers/gpu/drm/radeon/radeon_*.c

Fix this sparse warning:
  drivers/gpu/drm/radeon/r600_cp.c:1811:52: warning: Using plain integer as NULL pointer
  drivers/gpu/drm/radeon/radeon_cp.c:1363:52: warning: Using plain integer as NULL pointer
  drivers/gpu/drm/radeon/radeon_state.c:1983:61: warning: Using plain integer as NULL pointer

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/radeon_state.c b/drivers/gpu/drm/radeon/radeon_state.c
index ca00cef..fa728ec 100644
--- a/drivers/gpu/drm/radeon/radeon_state.c
+++ b/drivers/gpu/drm/radeon/radeon_state.c
@@ -1986,7 +1986,7 @@
 
 	/* find a virtual surface */
 	for (i = 0; i < 2 * RADEON_MAX_SURFACES; i++)
-		if (dev_priv->virt_surfaces[i].file_priv == 0)
+		if (dev_priv->virt_surfaces[i].file_priv == NULL)
 			break;
 	if (i == 2 * RADEON_MAX_SURFACES) {
 		return -1;