drm/i915/gt: Move CS interrupt handler to the backend

The different submission backends each have their own preferred
behaviour and interrupt setup. Let each handle their own interrupts.

This becomes more useful later as we to extract the use of auxiliary
state in the interrupt handler that is backend specific.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210521183215.65451-4-matthew.brost@intel.com