r600g: Don't return a value in function returning void.

Fixes this GCC warning.
radeon_state.c: In function 'radeon_state_fini':
radeon_state.c:140: warning: 'return' with a value, in function returning void

src/gallium/winsys/r600/drm/radeon_state.c

@@ -137,7 +137,7 @@ void radeon_state_fini(struct radeon_state *state)
 	unsigned i;
 
 	if (state == NULL)
-		return NULL;
+		return;
 	for (i = 0; i < state->nbo; i++) {
 		radeon_ws_bo_reference(state->radeon, &state->bo[i], NULL);
 	}