vc4: Power down the HVS when unused

[anholt]

We should disable SCALER_DISPCTRL's ENABLE bit (31) when we're not using any of the HVS channels. Make sure that all DISPCTRLn are disabled first, and have raised their EOF flags in IRQSTATUS. When we do this, most register reads of the block other than SCALER_DISPCTRL will stall the AXI bus, so make sure that /debug/dri/0/hvs* re-enables the display.