drm/i915/icl: create function to identify combophy port

This patch creates a function/wrapper to check if port is combophy port
instead of explicitly comparing ports.

Changes since V1:
 - keep all intel_port_is_* helper together (Lucas)

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Cc: Madhav Chauhan <madhav.chauhan@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004085043.10154-1-mahesh1.kumar@intel.com
(cherry picked from commit 176597a1

)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>