drm/i915/tc: Catch TC users accessing FIA registers without enable aux

As described in "drm/i915/tc/icl: Implement TC cold sequences" users
of TC functions should held aux power well during access to avoid
read garbage due HW in TC cold state.

v3:
- renamed is_tc_cold_blocked() to assert_tc_cold_blocked()
- restored the removed 0xffffffff checks

Reviewed-by: Imre Deak <imre.deak@intel.com>
Tested-by: You-Sheng Yang <vicamo.yang@canonical.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200414194956.164323-7-jose.souza@intel.com