drm/etnaviv: fix NULL check before some freeing functions is not needed

fixed the below warning:
drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c:84:2-8: WARNING: NULL check
before some freeing functions is not needed.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>