QHeaderView - no big update work if updates are disabled

QHeaderView - no big update work if updates are disabled

There is no reason to find out where to update the headerview
if updates aren't enabled.  Another approch I considered to skip the
whole calculation and just call update. I seriosly doubt that this
calculation and update of a particular QHeaderView rect will normally(*)
be faster than just updating everything.

However to be safe I have done the conservative fix.

(*) Normally but with many/fragmented spans in the headerview.

Change-Id: Ia812a747ee825653db0345cdc34f9d2f7155ea01
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>