drm/i915: Prepare for engine init unification

Move the execlist engine setup to vfuncs so that the engine
init loop is clearly split into the mode agnostic and
specific steps.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Chris Wilson <chris-wilson.co.uk>