hwdec_drmprime_drm: Fix a DRM buffer memory leakage

We use triple buffering for this interop and we were only unreffing the
data structures, which doesn't destroy the drm buffers.

This patch allows to make sure that we release the drm buffers on
playback end.

video/out/opengl/hwdec_drmprime_drm.c

@@ -83,8 +83,6 @@ static void set_current_frame(struct ra_hwdec *hw, struct drm_frame *frame)
     } else {
         memset(&p->current_frame.fb, 0, sizeof(p->current_frame.fb));
         mp_image_setrefp(&p->current_frame.image, NULL);
-        mp_image_setrefp(&p->last_frame.image, NULL);
-        mp_image_setrefp(&p->old_frame.image, NULL);
     }
 }
 
@@ -180,6 +178,9 @@ static int overlay_frame(struct ra_hwdec *hw, struct mp_image *hw_image,
                 }
             }
         }
+    } else {
+        while (p->old_frame.fb.fb_id)
+          set_current_frame(hw, NULL);
     }
 
     set_current_frame(hw, &next_frame);