drm/i915/dp: Remove unused DSC caps from intel_dp

The previous patches converted all users of the DSC DPCD caps to look
these up from the connector, so remove the version stored in intel_dp.

A follow-up patchset will read out the MST connector specific
capabilities in intel_dp_add_mst_connector() ->
intel_dp_mst_read_decompression_port_dsc_caps().

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

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/20231011171606.2540078-3-imre.deak@intel.com