drm: Add helpers for locking an array of BO reservations.

Now that we have the reservation object in the GEM object, it's easy
to provide a helper for this common case.  Noticed while reviewing
panfrost and lima drivers.  This particular version came out of v3d,
which in turn was a copy from vc4.

v2: Fix kerneldoc warnings.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190308161716.2466-2-eric@anholt.net
Acked-by: Rob Herring <robh@kernel.org> (v1)