drm/i915: Consolidate write_tail vfunc initializer

Introduce a function which initializes vfuncs mostly common
across engines and move write_tail initialization in it since
only one engine overrides the default.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>