drm/i915/suspend: replace I915_READ()/WRITE() with intel_de_read()/write()

Another straggler with I915_READ() and I915_WRITE() uses gone.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130111601.2817-6-jani.nikula@intel.com