drm/i915: Remove impossible tests for dev->dev_private

If we have a drm_device, we have a drm_i915_private (since they are the
same).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467711623-2905-3-git-send-email-chris@chris-wilson.co.uk


Reviewed-by: Matthew Auld <matthew.auld@intel.com>