drm/i915/tc: Fix up the legacy VBT flag only in disconnected mode

A follow-up patch simplifies the tc_cold_block()/unblock() functions,
dropping the power domain parameter. For this it must be ensured that
the power domain - which depends on the actual TC mode and so the VBT
legacy port flag - can't change while the PHY is in a connected state
and accordingly TC-cold is blocked. Make this so, by fixing up the VBT
legacy flag only in the disconnected TC mode, instead of whenever the
HPD state is retrieved.

Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323142035.1432621-15-imre.deak@intel.com