drm/i915/dp: Use connector DSC DPCD in intel_dp_dsc_get_slice_count()

Use the connector's DSC DPCD capabilities in
intel_dp_dsc_get_slice_count().

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/20231006133727.1822579-16-imre.deak@intel.com