xe_irq clean-up

https://lore.kernel.org/all/Y9hw0sSC58B32yPg@mdroper-desk1.amr.corp.intel.com/

...

diff --git a/drivers/gpu/drm/xe/xe_irq.c b/drivers/gpu/drm/xe/xe_irq.c

There's lots of "gen3," "gen11," etc. in this file that should probably
be cleaned up.

I believe some might still apply. But indeed let's clean that up at least
and also get ridden of 'gen'....