drm/etnaviv: Remove redundant NULL check

Fix the following coccicheck warnings:

./drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c:622:2-8: WARNING: NULL
check before some freeing functions is not needed.

./drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c:618:2-8: WARNING: NULL
check before some freeing functions is not needed.

./drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c:616:2-8: WARNING: NULL
check before some freeing functions is not needed.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>