drm/panfrost: fix GENERIC_ATOMIC64 dependency

On ARMv5 and earlier, a randconfig build can still run into

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

Rework the dependencies to always require a working cmpxchg64.

Fixes: db594ba3 ("drm/panfrost: depend on !GENERIC_ATOMIC64 when using COMPILE_TEST")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230117164456.1591901-1-arnd@kernel.org