vc4/hvs: Fix lbm size calculation for yuv

The code was reducing the number of components by one when we were not
blending with alpha. But that only makes sense if the components include
alpha.

For YUV, we were reducing the number of components for Y from one to zero
which resulted in no lbm space being allocated.

Fixes: https://github.com/raspberrypi/linux/issues/5912


Signed-off-by: Dom Cobley <popcornmix@gmail.com>