drm/i915/gt: Move engine setup out of set_default_submission

Now that we no longer switch back and forth between guc and execlists,
we no longer need to restore the backend's vfunc and can leave them set
after initialisation. The only catch is that we lose the submission on
wedging and still need to reset the submit_request vfunc on unwedging.

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/20210521183215.65451-2-matthew.brost@intel.com