drm/i915/selftests: Run vma tests only if current->mm is present

drm/i915/selftests: Run vma tests only if current->mm is present

This set of tests require userspace memory to map objects, so
run them only if this that memory is available.

Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Reviewed-by: Michał Grzelak <michal.grzelak@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/20260508080214.1979686-3-krzysztof.karas@intel.com