drm/i915/display: Move out code to return the digital_port of the aux ch

Moving the code to return the digital port of the aux channel also
removing the intel_phy_is_tc() to make it generic.
digital_port will be needed in icl_tc_phy_aux_power_well_enable()
so adding it as a parameter to icl_tc_port_assert_ref_held().

While at at removing the duplicated call to icl_tc_phy_aux_ch() in
icl_tc_port_assert_ref_held().

v2:
- fixed build when DRM_I915_DEBUG_RUNTIME_PM is not set
- moved to before hsw_wait_for_power_well_enable() as it will be
needed by hsw_wait_for_power_well_enable() in a future patch

v4:
- fixed action of if (!dig_port), continue instead of return

Cc: You-Sheng Yang <vicamo@gmail.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/20200414194956.164323-1-jose.souza@intel.com