drm/etnaviv: add further minor features and varyings count

Export further minor feature bitmasks and the varyings count from
the GPU specifications registers to userspace.

Acked-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>