drm/etnaviv: constify etnaviv_iommu_ops structures

Declare etnaviv_iommu_ops structure as const as it is only used when
the reference of one of its field is stored in the ops field of a
iommu_domain structure. This ops field is of type const, so
etnaviv_iommu_ops structures having similar properties can be declared
const too.

Done using Coccinelle.
Before and after size details of .o file remains the same after
cross compiling for arm architecture.

lst: Trimmed commit message, apply the same change to iommu_v2.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>