drm: Add helpers for setting up an array of dma_fence dependencies.

I needed to add implicit dependency support for v3d, and Rob Herring
has been working on it for panfrost, and I had recently looked at the
lima implementation so I think this will be a good intersection of
what we all want and simplify our scheduler implementations.

v2: Rebase on xa_limit_32b API change, and tiny checkpatch cleanups on
    the way in (unsigned int vs unsigned, extra return before
    EXPORT_SYMBOL_GPL)

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190401222635.25013-6-eric@anholt.net
Reviewed-and-tested-by: Qiang Yu <yuq825@gmail.com> (v1)