drm/i915: Fix incorrect comment

While the comment singles out Port A or B, the code says Port A or *D*.
Looking at the history it seems that the comment was added after the code,
so it seems likely that the code is correct, not the comment.

CC: Jani Nikula <jani.nikula@intel.com>
CC: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: David Weinehall <david.weinehall@linux.intel.com>
Reviewed-by: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180209130755.11893-1-david.weinehall@linux.intel.com