drm/i915/tc: Refactor TC-cold block/unblock helpers

drm/i915/tc: Refactor TC-cold block/unblock helpers

A follow-up change will select the TC-cold blocking power domain based
on the TypeC mode, prepare for that here.

Also bring intel_tc_cold_requires_aux_pw() earlier to its logical place
for readability.

No functional change.

v2: Add code comment about IOM reg accesses in TCCOLD. (Jose)

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/20210929132833.2253961-4-imre.deak@intel.com