iommu/arm-smmu: Remove redundant calculation of gr0 base address

Since commit 1463fe44fd0f ("iommu/arm-smmu: Don't use VMIDs for stage-1
translations"), we don't need the GR0 base address when initialising a
context bank, so remove the useless local variable and its init code.

Signed-off-by: Will Deacon <will.deacon@arm.com>