I'm not sure why it doesn't have an effect on my system. When I add the i915.conf to /etc/modprobe.d/ and reboot my system the PSR is still active:

    [root@fwk ~]# cat /etc/modprobe.d/i915.conf
    options i915 enable_psr=0
    [root@fwk ~]# cat /sys/kernel/debug/dri/1/i915_edp_psr_status
    Sink support: yes [0x03]
    PSR mode: PSR2 enabled
    Source PSR ctl: enabled [0x80000226]
    Source PSR status: SLEEP [0x30000114]
    Busy frontbuffer bits: 0x00000000
    Frame: PSR2 SU blocks:
    0 0
    1 0
    2 0
    3 0
    4 0
    5 0
    6 0
    7 0

It also has no effect when I append i915.enable_psr=0 to GRUB_CMDLINE_LINUX in /etc/default/grub and reboot.