msdkmjpegdec: support 422 output

MSDK supports JPEG YUY2 (422 chroma) output color
format. The color format of input bitstream is
described by JPEGChromaFormat and JPEGColorFormat
fields in the mfxInfoMFX structure which is filled
in by the MFXVideoDECODE_DecodeHeader function.

To obtain lossless decoded output from 422 encoded
JPEGs, we must set the output color format in the
FourCC and ChromaFormat fields in the mfxFrameInfo
structure to the appropriate values at post_configure
so that they are propagated through to the srcpad
caps accordingly.