drm/i915/tc: Add TC PHY hook to get the TC-cold blocking power domain

Instead of the corresponding if ladder, add a TC PHY hook to get the
platform and TC mode specific power domain used for blocking the TC-cold
power state.

Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323142035.1432621-20-imre.deak@intel.com