drm/i915/gt: finish INTEL_GEN and friends conversion

Commit c816723b ("drm/i915/gt: replace IS_GEN and friends with
GRAPHICS_VER") converted INTEL_GEN and friends to the new version check
macros.  Meanwhile, some changes sneaked in to use INTEL_GEN. Remove the
last users so we can remove the macros.

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/20210707181325.2130821-2-lucas.demarchi@intel.com