drm/vc4: Configure the HVS COB allocations

The HVS Composite Output Buffer (COB) is the memory used to
generate the output pixel data.
Until now the vc4 driver has been relying on the firmware to
have set these to sensible values.

In testing triple screen support it has been noted that only
1 line was being assigned to HVS channel 2. Whilst that is fine
for the transposer (TXP), and indeed needed as only some pixels
have an alpha channel, it is insufficient to run a live display.

Split the COB more evenly between the 3 HVS channels.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>

Revert vc4_regs change