gpu/drm/i915: nuke old GEN macros

Now that all the codebase is converted to the new *VER macros, remove
the old GEN ones.

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/20210707181325.2130821-4-lucas.demarchi@intel.com