Mapping the imported pages of a DMA-buf into an userspace process
doesn't work as expected.

But we have reoccurring requests on this approach, so split the
functions for this and  document that dma_buf_mmap() needs to be used
instead.

v2: split it into two functions
v3: rebased on latest changes
v4: update commit message a bit
Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/403838/