M2: camera: videobuf2-core: make HACK less HACK-ish

This makes __qbuf_userptr usable again.  The particular if that's
commented out is not present in 3.0, and could possibly fail there as
well.

Failure now is in vfe irq.

08-07 06:37:40.676 ERROR: irq	errorIrq
08-07 06:37:40.678 ERROR: vfe32_irq: camif errors
08-07 06:37:40.682 ERROR: camifStatus  = 0xa00
08-07 06:37:40.685 ERROR: msm_mctl_reserve_free_buf: stream is turned
off