drm/i915/tc: Add/use helpers to retrieve TypeC port properties

Instead of directly accessing the TypeC port internal struct members,
add/use helpers to retrieve the corresponding properties.

No functional change.

Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210921002313.1132357-6-imre.deak@intel.com