r300g: Silence 'control reaches end of non-void function' warning.

Fixes this GCC warning.
r300_hyperz.c: In function 'r300_get_hiz_func':
r300_hyperz.c:65: warning: control reaches end of non-void function

src/gallium/drivers/r300/r300_hyperz.c

@@ -61,6 +61,10 @@ static enum r300_hiz_func r300_get_hiz_func(struct r300_context *r300)
     case PIPE_FUNC_GREATER:
     case PIPE_FUNC_GEQUAL:
         return HIZ_FUNC_MIN;
+
+    default:
+        assert(0);
+        return HIZ_FUNC_NONE;
     }
 }