Add test_mode = 3 that add VPU_JOB_FLAGS_NULL_SUBMISSION_MASK
flag to the job send to the VPU device. Then the VPU will process
the job but won't execute commands (except the command to signal
the fence).

This can be used to estimate job processing overhead in the host
software and VPU firmware.

Unlike the null hardware mode, the null submission mode will
still work even if UMD uses VPU fences to track job completion.
Signed-off-by: Andrzej Kacprowski <andrzej.kacprowski@linux.intel.com>
Reviewed-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231028133415.1169975-6-stanislaw.gruszka@linux.intel.com