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: db594ba3fcf9 ("drm/panfrost: depend on !GENERIC_ATOMIC64 when using COMPILE_TEST")
Signed-off-by: Arnd Bergmann <[e-mail]>
Reviewed-by: Steven Price <[e-mail]>
Signed-off-by: Steven Price <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/panfrost/Kconfig

···
 config DRM_PANFROST
 	tristate "Panfrost (DRM support for ARM Mali Midgard/Bifrost GPUs)"
 	depends on DRM
-	depends on ARM || ARM64 || (COMPILE_TEST && !GENERIC_ATOMIC64)
+	depends on ARM || ARM64 || COMPILE_TEST
+	depends on !GENERIC_ATOMIC64    # for IOMMU_IO_PGTABLE_LPAE
 	depends on MMU
 	select DRM_SCHED
 	select IOMMU_SUPPORT