drm/etnaviv: enable debug registers on demand

Some performance register are debug register and they need to
be enabled in order to be functional.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>