drm/vc4: Reformat and the binner bo allocation helper

In preparation for wrapping the binner bo allocation helper with
put/get helpers, pass the vc4 dev directly and drop the vc4 prefix.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190516145544.29051-2-paul.kocialkowski@bootlin.com