drm/i915: switch vlv_suspend to use intel uncore register accessors

Prefer intel_uncore_* over I915_READ, I915_WRITE, and POSTING_READ.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200212144058.5686-2-jani.nikula@intel.com