drm/vc4: Check for the binner bo before handling OOM interrupt

Since the OOM interrupt directly deals with the binner bo, it doesn't
make sense to try and handle it without a binner buffer registered.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190516145544.29051-4-paul.kocialkowski@bootlin.com