drm/etnaviv: ignore VG GPUs with FE2.0

Ignore GPUs with a 2.0 front end.  These have a different register
layout for the front end, which provokes imprecise aborts from the
register accesses in the 'gpu' debugfs file.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>