drm/i915: Move allocation of various workqueues earlier during init

Workqueue initalization doesn't depend on any other device specific
resource, so move it close to the beginning, so we don't need to
consider them when thinking about dependencies for other resources.

Also factor out things to separate init/cleanup functions to make
i915_driver_load()/unload() clearer, atm it's somewhat difficult to
follow there in what order resources are inited/cleaned-up.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: David Weinehall <david.weinehall@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453209992-25995-6-git-send-email-imre.deak@intel.com