drm/i915: Expose RPS thresholds in sysfs

User feedback indicates significant performance gains are possible in
specific games with non default RPS up/down thresholds.

Expose these tunables via sysfs which will allow users to achieve best
performance when running games and best power efficiency elsewhere.

Note this patch supports non GuC based platforms only.

v2:
 * Make checkpatch happy.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
References: https://gitlab.freedesktop.org/drm/intel/-/issues/8389
Cc: Rodrigo Vivi <rodrigo.vivi@kernel.org>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230717164013.826614-4-tvrtko.ursulin@linux.intel.com