drm/i915: Pull the vgpu uncore funcs apart from the rest of gen6+

I need to add a new variable into GEN6_{READ,WRITE}_HEADER, but the vgpu
won't need it, so let's avoid an unused variable warning by splitting
the vgpu stuff to use its own macros.

Cc: Eddie Dong <eddie.dong@intel.com>
Cc: Jike Song <jike.song@intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Yu Zhang <yu.c.zhang@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: Zhiyuan Lv <zhiyuan.lv@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446672017-24497-26-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Zhiyuan Lv <zhiyuan.lv@intel.com>