drm/i915/tc: Drop tc_cold_block()/unblock()'s power domain parameter

Simplify tc_cold_block()/unblock() by dropping their power domain
parameter. The power domain depends on the current TC mode, which -
after the previous patch - can't change while the PHY is connected,
holding a TC-cold-off power domain reference. Based on this the domain
can be deducted from the current TC mode instead of having to pass this
as a parameter.

Blocking TC-cold for the PHY HW readout happens before the current TC
mode is determined, so here the initial power domain must be still
manually passed.

For debugging still keep track of the domain used for tc_cold_block()
and verify that it remained the same until tc_cold_unblock().

While at it rename tc_cold_get_power_domain() to
tc_phy_cold_off_domain(), reflecting the name of platform specific hook
added in the next patch.

Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323142035.1432621-19-imre.deak@intel.com