Support `HDR_OUTPUT_METADATA` on DP MST encoders

For connectors using DP MST encoders, the HDR_OUTPUT_METADATA property isn't exposed, thus it's not possible to set. In practice, this means that monitors connected via Thunderbolt can't enter HDR mode.

Stuff that would need figuring out:

• Do we need to do something extra (allocate bandwidth/etc.) in the common MST code for SDP transmission?
• Rework the "infoframe" code to work per-MST stream (currently it's tied into the SST encoder)
• Probably some other stuff I've not thought of yet

Should probably mention here that we also lack MST deep color support. HDR probably isn't going to be great without that. And implementing that will require us to properly manage the MST link bandwidth to make sure each stream gets a reasonable share, instead of the current approach where the first stream to be enabled steals as much as it wants, potentially leaving the other streams with insufficient bandwidth.

added feature: display/DP MST label

mentioned in issue pq/color-and-hdr#9 (closed)

mentioned in issue #8340 (closed)

added Community label

@ideak @vsyrjala @uma_mnnit @nvsrikanth Since MST link bandwidth management in place, should we take this task ?