drm/i915: use dev_priv directly in gen8_ppgtt_notify_vgt

Remove dev local and use to_i915() in gen8_ppgtt_notify_vgt.

v2: use dev_priv directly for QUESTION_MACROS (Joonas Lahtinen)

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461323365-21256-1-git-send-email-matthew.auld@intel.com