tracing: Separate hist state updates from hist registration

hist_register_trigger() handles both new hist registration as well as
existing hist registration through event_command.reg().

Adding a new function, existing_hist_update_only(), that checks and
updates existing histograms and exits after doing so allows the
confusing logic in event_hist_trigger_parse() to be simplified.

Signed-off-by: Tom Zanussi <zanussi@kernel.org>