drm/i915/psr: Unify pre/post hooks

pre/post hooks are doing things differently. Unify them.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230329150703.432072-2-jouni.hogander@intel.com