dma-buf: Documentation update for Kconfig select

As per Linus' comment, dma-buf Kconfig entry shouldn't have an option
text, but should be selected by the subsystems that use it.

Add this information in the documentation as well.

Signed-off-by: Sumit Semwal <sumit.semwal@ti.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>