drm/i915/gvt: replace I915_WRITE with intel_uncore_write

Let's avoid adding new I915_WRITE uses while we try to get rid of them.

Fixes: 5f60b12e ("drm/i915/gvt: Save/restore HW status to support GVT suspend/resume")
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Hang Yuan <hang.yuan@linux.intel.com>
Cc: Colin Xu <colin.xu@intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: intel-gvt-dev@lists.freedesktop.org
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130111601.2817-9-jani.nikula@intel.com