vc4: Correct lbm size and calculation

LBM base address is measured in units of pixels per cycle.
That is 4 for 2711 (hvs5) and 2 for 2708.

We are wasting 75% of lbm by indexing without the scaling.
But we were also using too high a size for the lbm resulting
in partial corruption (right hand side) of vertically
scaled images, usually at 4K or lower resolutions with more layers.

The physical RAM of LBM on 2711 is 8 * 1920 * 16 * 12-bit
(pixels are stored 12-bits per component regardless of format).

The LBM adress indexes work in units of pixels per clock,
so for 4 pixels per clock that means we have 32 * 1920 = 60K

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