drm/i915/gt: s/gen3/gen2/

Now that we use the gen3 codepaths also for gen2
rename everything to gen2_ to match.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241008214349.23331-3-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>