drm/i915/psr: clean up PSR debugfs sink status error handling

Handle errors first and return early, and reduce indentation on the
happy day code path.

Cc: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230317134144.223936-3-jani.nikula@intel.com