dma-buf: Fix pin callback comment

The pin callback does not necessarily have to move the memory to system
memory, remove the sentence from the comment.

Link: https://lore.kernel.org/r/20211012120903.96933-2-galpress@amazon.com


Signed-off-by: Gal Pressman <galpress@amazon.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>