tests/kms_concurrent: For i915 devices run allocator in multiprocess mode

Test calls igt_fork() so for i915 requires offset allocation arbitration
(allocator in multiprocess mode) especially when same drm fd is used
in children. Dedicated thread (intel_allocator_multiprocess_start())
is required to be started on the very beginning to handle offset
allocations as well as stopping it (intel_allocator_multiprocess_stop())
before test exits.

Signed-off-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Cc: Luciano Coelho <luciano.coelho@intel.com>
Cc: Swati Sharma <swati2.sharma@intel.com>
Acked-by: Petri Latvala <petri.latvala@intel.com>
Tested-by: Luciano Coelho <luciano.coelho@intel.com>
23 jobs for master in 5 minutes and 31 seconds (queued for 2 seconds)