drm/i915/icl: replace check for combo phy

These are the only places that assume ports A and B are the ones with
combo phy.  Let's use intel_port_is_combophy() there to make sure
it checks for combo phy ports the same way everywhere.

v2: define for_each_combo_port() helper to check the ports

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181114011509.3667-1-lucas.demarchi@intel.com