drm/i915/display: Add intel_legacy_aux_to_power_domain()

This is a similar function to intel_aux_power_domain() but it do not
care about TBT ports, this will be needed by ICL TC sequences.

v2:
- renamed to intel_legacy_aux_to_power_domain()

Cc: Imre Deak <imre.deak@intel.com>
Cc: Cooper Chiou <cooper.chiou@intel.com>
Cc: Kai-Heng Feng <kai.heng.feng@canonical.com>
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/20200414194956.164323-2-jose.souza@intel.com