drm/panfrost: depend on !GENERIC_ATOMIC64 when using COMPILE_TEST

Since panfrost has a 'select' on IOMMU_IO_PGTABLE_LPAE we must depend on
the same set of flags. Otherwise IOMMU_IO_PGTABLE_LPAE will be forced on
even though it cannot build (no support for cmpxchg64).

This fixes the following warning from kconfig:

WARNING: unmet direct dependencies detected for IOMMU_IO_PGTABLE_LPAE
  Depends on [n]: IOMMU_SUPPORT [=y] && (ARM || ARM64 || COMPILE_TEST [=y] && !GENERIC_ATOMIC64 [=y])
  Selected by [y]:
  - DRM_PANFROST [=y] && HAS_IOMEM [=y] && DRM [=y] && (ARM || ARM64 || COMPILE_TEST [=y]) && MMU [=y]

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190417152928.10790-1-steven.price@arm.com