kms/crtc: Fix gamma state leak

The gamma value pointers of the current_state are overwritten by the
calls to memdup causing a small leak. while the leak itself is small, it
can be triggered quite often from things like night light.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/1020

src/backends/native/meta-kms-crtc.c

@@ -203,6 +203,7 @@ meta_kms_crtc_predict_state (MetaKmsCrtc   *crtc,
       if (gamma->crtc != crtc)
         continue;
 
+      clear_gamma_state (crtc);
       crtc->current_state.gamma.size = gamma->size;
       crtc->current_state.gamma.red =
         g_memdup (gamma->red, gamma->size * sizeof (uint16_t));