drm/i915/ringbuffer: Move all generic engine->dispatch_batchbuffer together

drm/i915/ringbuffer: Move all generic engine->dispatch_batchbuffer together

Consolidate the block of default vfuncs for dispatching the batchbuffer.
Just a minor tweak on top of Tvrtko's great job of tidying up the vfunc
initialisation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467361093-20209-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>