QSGRenderer: sanity check attribute regs only when QSG_SANITY_CHECK=1

The sanity check that attribute registers are disabled that was always
done in debug mode took roughly 16ms on my machine and caused a trivial
QML animation to eat 100% CPU.

Task-number: QTBUG-35443
Change-Id: I8ba2a80db341d17e08216b3cad1678dd59b9b1a5
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>