drm/i915: Pull out some more common engine init code

Created two common helpers for engine setup and engine init phases
respectively to help with code sharing.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1468422221-12132-1-git-send-email-tvrtko.ursulin@linux.intel.com
Reviewed-by: Chris Wilson <chris-wilson.co.uk>