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 <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]