drm/etnaviv: Read some FE registers twice

On some hardware (such at the GC7000 rev 6009), these registers need to be
read twice to return the correct value. Hide that in gpu_read().

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>