Refactor the vp9_get_frame code for frame parallel.

In frame parallel decoding mode,  there will be still several frames inside
the decoder when application stop calling vpx_codec_decode to decode frames.
The application will need to keep calling vpx_codec_get_frame to get all the
remaining decoded frames in the decoder.

Change-Id: I2ce8260a91282f045bb9a6093ff8a606b1990f14