OMXVideoDecoderBase: call renderDone in processOutPortBuffers

it is incorrect to call renderDone() in PreFillBuffer.
    a) rendering hasn't been done yet
    b) two threads may come here twice:
        there are ComponentBase::Work->OMXVideoDecoderBase::getDecodedBuffer and ComponentBase::CBaseFillThisBuffer
        calling renderDone() twice will confuse codec, since codec usually recycle surface buffer upon this call.

clear pPlatformPrivate after renderDone(), since it has been returned back to codec.
it disclose the issue of flagNativeBuffer in ProcessorPreFillBuffer, fix it.

Change-Id: I48222fd769ff7cc5c5cf8591c89b51ffff4b0721