v4l2videodec: Fix activation of internal pool

If the driver does not support VIDIOC_CREATE_BUFS ioctl, the pool
configuration may get changed, which requires a validation. This would
fail to activate a pool in a case it shouldn't normally fail unless we
are out of memory.
24 jobs for !2456 with v4l2videodec-pool-activation-fix in 34 seconds (queued for 2 seconds)
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