drm/i915/psr: Fix the delayed vblank w/a

Fix the code to correctly determine whether delayed vblank
is used or not.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230127173044.24108-3-ville.syrjala@linux.intel.com


Reviewed-by: Jouni Högander <jouni.hogander@intel.com>