perf scripts python: export-to-postgresql.py: Fix DROP VIEW power_events_view

PostgreSQL can error if power_events_view is not dropped before its
dependent tables e.g.

  Exception: Query failed: ERROR:  cannot drop table mwait because other
  objects depend on it
  DETAIL:  view power_events_view depends on table mwait

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Fixes: aba44287

 ("perf scripts python: export-to-postgresql.py: Export Intel PT power and ptwrite events")
Link: http://lkml.kernel.org/r/20190708055232.5032-2-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>