libdm: report: postpone printing of JSON starting '{' character till it's needed

libdm: report: postpone printing of JSON starting '{' character till it's needed

This patch moves printing of starting '{' character for JSON output up
untili it's known there's any further output following - either the
content or ending '}' character.

Also, remove unnecessary switch for different report group types and
calling individual functions to handle dm_report_group_create as that
code is shared for all existing types at the moment.