drm/etnaviv: Add fdinfo support for memory stats

Use the new helper to export stats about memory usage.

Signed-off-by: Christian Gmeiner <cgmeiner@igalia.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>