drm/vc4: Check for V3D before binner bo alloc

Check that we have a V3D device registered before attempting to
allocate a binner buffer object.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190516145544.29051-3-paul.kocialkowski@bootlin.com