omxvideoenc: let encoder base class handle ALLOCATION query

Fixing a regression introduced in my previous patch
(7c40a91c31).

The ALLOCATION query needs to be handled by GstVideoEncoder (to call
propose_allocation()) so chain up the query handling rather than early
returning.

omx/gstomxvideoenc.c

@@ -3313,7 +3313,6 @@ gst_omx_video_enc_sink_query (GstVideoEncoder * encoder, GstQuery * query)
           GST_QUERY_TYPE_NAME (query));
 
       gst_omx_video_enc_drain (self);
-      return TRUE;
     }
     default:
       break;