gl_drm: Release any unqueued buffer before starting a render

If there's a "next" buffer set, we're better off freeing it before we
start to render.  This lets us keep gbm from allocating an extra buffer
which it currently never frees.

src/modules/evas/engines/gl_drm/evas_outbuf.c

@@ -583,6 +583,7 @@ evas_outbuf_reconfigure(Outbuf *ob, int w, int h, int rot, Outbuf_Depth depth)
 Render_Engine_Swap_Mode
 evas_outbuf_buffer_state_get(Outbuf *ob)
 {
+   ecore_drm2_fb_release(ob->priv.output, EINA_FALSE);
    /* check for valid output buffer */
    if (!ob) return MODE_FULL;