drm/etnaviv: potential NULL dereference

The etnaviv_gem_prime_get_sg_table() is supposed to return error
pointers.  Otherwise it can lead to a NULL dereference when it's called
from drm_gem_map_dma_buf().

Fixes: 5f4a4a73 ("drm/etnaviv: fix gem_prime_get_sg_table to return new SG table")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>