drm/etnaviv: fold __etnaviv_gem_new into caller

This function has only one caller and it isn't expected that there will
be any more in the future. Folding this function into the caller is
helping the readability.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>