drm/i915: Update i915_gem_get_ggtt_size/_alignment to use drm_i915_private

For consistency, internal functions should take drm_i915_private rather
than drm_device. Now that we are subclassing drm_device, there are no
more size wins, but being consistent is its own blessing.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470324762-2545-12-git-send-email-chris@chris-wilson.co.uk