I believe newer kernels have PSR enabled by default. It can be force disabled by adding this kernel boot parameter i915.enable_psr=0 and force enabled with i915.enable_psr=1
PSR status can be checked with: cat /sys/kernel/debug/dri/0/i915_edp_psr_status
Extra info:
Sidenote, just read this thread on Fedora 35 that confirms PSR works out of the box: