drm/i915: Fix the DDI encoder names

drm/i915: Fix the DDI encoder names

I totally fumbled the ?: usage when generating the DDI encoder
names. Reverse the things that need reversing, and to make it
a bit less messy add a few macros to hide the arithmetic on the
port enums.

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Fixes: 2d709a5a624c ("drm/i915: Give DDI encoders even better names")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201117154028.8516-1-ville.syrjala@linux.intel.com
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>