drm: share address space for dma bufs

Use the shared address space of the drm device (see drm_open() in
drm_file.c) for dma-bufs too.  That removes a difference betweem drm
device mmap vmas and dma-buf mmap vmas and fixes corner cases like
dropping ptes (using madvise(DONTNEED) for example) not working
properly.

Also remove amdgpu driver's private dmabuf update.  It is not needed
any more now that we are doing this for everybody.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20191127092523.5620-3-kraxel@redhat.com