drm/i915: Record default rps threshold values

Record the default values as preparation for exposing the sysfs controls.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
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/20230717164013.826614-2-tvrtko.ursulin@linux.intel.com