drm/i915/tc: Add asserts in TC PHY hooks that the required power is on

Add an assert to each TC PHY hook that their required power domain is
enabled.

While at it add a comment describing the domains used on each platform
and TC mode.

v2: Fix non kernel-doc multiline comments. (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323142035.1432621-21-imre.deak@intel.com