drm/etnaviv: make local symbols static

Fixes the following sparse warnings:

drivers/gpu/drm/etnaviv/etnaviv_iommu.c:161:39: warning:
 symbol 'etnaviv_iommuv1_ops' was not declared. Should it be static?
drivers/gpu/drm/etnaviv/etnaviv_iommu_v2.c:239:39: warning:
 symbol 'etnaviv_iommuv2_ops' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>