drm/i915/psr: Split setting sff and cff bits away from intel_psr_force_update

This is a clean-up and a preparation for adding own SFF and CFF registers
for LunarLake onwards.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250213064804.2077127-4-jouni.hogander@intel.com