dma-buf: Update obsoluted comments on dma_buf_vmap/vunmap()

A comment for the dma_buf_vmap/vunmap() is not catching up a
corresponding implementation.

Signed-off-by: Shunsuke Mie <mie@igel.co.jp>
Link: https://patchwork.freedesktop.org/patch/msgid/20211008112009.118996-1-mie@igel.co.jp


Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>