drm/vc4: Make vc4_lbm_size() return 0 when vertical scaling is disabled

LBM is not needed when vertical scaling is disabled. Return 0 in this
case to avoid allocating LBM memory that will anyway be unused.

While at it, drop the test on ->is_unity which is now redundant.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181130090254.594-1-boris.brezillon@bootlin.com