va: decoder: Do not only consider chroma when get preferred format.

The current way of preferred format selection is not complete. It only choose the formats which has the same chroma type with the decoder's rt_format. The rt_format tells the decoder which chroma type the stream is, but it is not decisive for the output surface format. For example, main-10 profile of HEVC can also contain and decode 8 bits stream. But that profile/entrypoint pair just support P010_10LE as the output format. So we fail all these cases because format selection is UNKNOWN.

The better way should be that we consider the chroma first, and choose a default one in our decoder's src caps if we can not find one.