drm/msm: Stop passing vm to msm_framebuffer

The fb only deals with kms->vm, so make that explicit.  This will start
letting us refcount the # of times the fb is pinned, so we can only
unpin the vma after last user of the fb is done.  Having a single
reference count really only works if there is only a single vm.

Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Tested-by: Antonino Maniscalco <antomani103@gmail.com>
Reviewed-by: Antonino Maniscalco <antomani103@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/661476/