drm/i915/icl: Refactor get_ddi_pll using helper func

Use the existing port-to-id helper function, to refactor
hence making it scalable.

Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Madhav Chauhan <madhav.chauhan@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/20181003072203.12848-4-mahesh1.kumar@intel.com