omxvideoenc: drain encoder on ALLOCATION and DRAIN queries

omxvideoenc: drain encoder on ALLOCATION and DRAIN queries

Ensure that the encoder releases all its input buffers when requested by
upstream. Encoder input buffers may be shared with downstreaming (when
using dmabuf), upstream may then request the encoder to
drain when reconfiguring before destroying its buffers.

Also drain on ALLOCATION query as we already do in kmssink as that
notify of a format change.

Fix "decoder ! encoder" pipeline when decoding a file with different
resolutions on Zynq.