Add mono_chrome to VAEncSequenceParameterBufferAV1

[rosetta-jpn]

To chroma subsampling completely, it is required to set mono_chrome.
See https://aomediacodec.github.io/av1-spec/#color-config-semantics

In fact, VADecPictureParameterBufferAV1 has mono_chrome variable, but VAEncSequenceParameterBufferAV1 doesn't have.
It is necessary to add mono_chrome to VAEncSequenceParameterBufferAV1.

[rosetta-jpn]

Friendly ping?

[XinfengZhang]

@rosetta-jpn because current intel implementation does not support it, but indeed , maybe need add it for common usage, do you know which vender are using it? will you file an PR to add it?

[rosetta-jpn]

I think no vendor supports mono_chrome today. It should be there for completeness.
I will add it.

[rosetta-jpn]

Created pull request #697