drm/etnaviv: NULL vs IS_ERR() buf in etnaviv_core_dump()

The etnaviv_gem_get_pages() never returns NULL.  It returns error
pointers on error.

Fixes: a8c21a54 ("drm/etnaviv: add initial etnaviv DRM driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>