drm/etnaviv: initialize idle mask before querying the HW db

If there is a match in the HW DB, the function is left early, before
inititalizing the idle mask. Fix this by doing the init earlier, as
only old GPUs, not present in the HW DB need a different idle mask.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>