drm/i915: Fix AUX power domain toggling across TypeC mode resets

Make sure to select the port's AUX power domain while holding the TC
port lock. The domain depends on the port's current TC mode, which may
get changed under us if we're not holding the lock.

This was left out from
commit 8c10e226 ("drm/i915: Keep the TypeC port mode fixed for detect/AUX transfers")

Cc: <stable@vger.kernel.org> # v5.4+
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/20200514204553.27193-1-imre.deak@intel.com