drm/i915: Add helpers for managing rps thresholds

In preparation for exposing via sysfs add helpers for managing rps
thresholds.

v2:
 * Force sw and hw re-programming on threshold change.

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/20230717164013.826614-3-tvrtko.ursulin@linux.intel.com