[media] vb2: remove TODO comment for dma-buf in QBUF

There is a TODO comment about the dma-buf being mapped in VIDIOC_QBUF
instead of doing it closer to when the actual DMA is going to happen
when the buffers are queued in the driver (i.e: __enqueue_in_driver).

But there is a reason to do it earlier in QBUF, and is that userspace
has no way to know if a exported dma-buf can be imported successfully
and so relies on QBUF succeeding as indication that the dma-buf mapped.

If QBUF fails, the application can fallback to another streaming I/O
method. But moving the dma-buf mapping later when queueing the buffers
can be too late for userspace to recover, since it may had dropped the
buffer(s) already when it knows that the dma-buf mapping failed.

So remove the TODO instead and change the comment to explain this.

Suggested-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>