drm/etnaviv: consolidate hardware fence handling in etnaviv_gpu

This is the only place in the driver that should have to deal with
the raw hardware fences. To avoid any further confusion, consolidate
the fence handling in this file and remove any traces of this from
the header files.

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