drm/etnaviv: fix etnaviv_cmdbuf_suballoc_new return value

The call site expects to get either a valid suballoc or an error
pointer, so a NULL return will not be treated as an error. Make
sure to always return a proper error pointer in case something goes
wrong.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Guido Günther <agx@sigxcpu.org>