drm/i915/display: replace IS_GEN() in commented code

Since we are replacing IS_GEN() with GRAPHICS_VER(), make sure we take
care of the comments as well.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210603165428.3625495-8-lucas.demarchi@intel.com