drm/i915/dp: Store DSC DPCD capabilities in the connector

In an MST topology the DSC capabilities are specific to each connector,
retrieved either from the sink if it decompresses the stream, or from a
branch device between the source and the sink in case this branch device
does the decompression. Accordingly each connector needs to cache its
own DSC DPCD and FEC capabilities, along with the AUX device through
which the decompression can be enabled. This patch prepares for that by
storing the capabilities and the DSC AUX device in the connector, for
now these just matching the version stored in intel_dp. The follow-up
patches will convert all users to look up these in the connector instead
of intel_dp, after which the intel_dp copies are removed.

v2:
- Rebased on intel_edp_get_dsc_sink_cap() addition in previous patch.
v3:
- Rebased on read-out fix for eDP in previous patch.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com> (v1)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231011171606.2540078-2-imre.deak@intel.com