drm: Add a drm_gem_objects_lookup helper

Similar to the single handle drm_gem_object_lookup(),
drm_gem_objects_lookup() takes an array of handles and returns an array
of GEM objects.

v2:
- Take the userspace pointer directly and allocate the array.
- Expand the function documentation.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Acked-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190409205427.6943-3-robh@kernel.org