drm/etnaviv: use defined constants for the chip model

Use the defined constants in common.xml.h for the chip model rather
than coding these as hex numbers.

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>