drm/i915/bios: reduce the scope of some local variables in parse_ddi_port()

No functional changes.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180411131519.9091-2-jani.nikula@intel.com