i915: Silence unused variable warning in non-debug builds.

Fixes this GCC warning.
i830_vtbl.c: In function 'i830_assert_not_dirty':
i830_vtbl.c:704: warning: unused variable 'i830'

src/mesa/drivers/dri/i915/i830_vtbl.c

@@ -703,6 +703,7 @@ i830_assert_not_dirty( struct intel_context *intel )
 {
    struct i830_context *i830 = i830_context(&intel->ctx);
    assert(!get_dirty(&i830->state));
+   (void) i830;
 }
 
 static void