drm/i915/debugfs: replace I915_READ() with intel_uncore_read()

Another straggler with I915_READ() uses gone.

Arguably some of these should use intel_de_read(), however not
all. Prioritize I915_READ() removal in general over migrating to the
pedantically correct replacement right away.
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130111601.2817-5-jani.nikula@intel.com