drm/i915/tc: Get power ref for reading the HPD live status register

Enable the power required for the HPD live status register access
instead of depending on the caller blocking the TC-cold power state
(during HW readout and connector probing).

A follow up patch will remove connecting/disconnecting the PHY around
connector probing, so querying the HPD status can happen in this case
without TC-cold being blocked.

Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323142035.1432621-24-imre.deak@intel.com