drm/i915/debugfs: remove RPS autotuning details from i915_rps_boost_info

The information is no longer relevant, so remove it. This also removes
the last users of I915_READ_FW()

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
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/20201130111601.2817-1-jani.nikula@intel.com