drm/i915/gt: Pull engine w/a initialisation into common

We need to setup the workarounds on all engines, with the knowledge
about which platforms each workaround applies to kept together in the
workaround list. As such, we can pull the w/a initialisation into the
common setup and try to avoid duplicating knowledge about when to setup
the workarounds.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190703135805.7310-2-chris@chris-wilson.co.uk