drm/vc4: Set AXI panic modes for the HVS

The HVS can change AXI request mode based on how full the COB
FIFOs are.
Until now the vc4 driver has been relying on the firmware to
have set these to sensible values.

With HVS channel 2 now being used for live video, change the
panic mode for all channels to be explicitly set by the driver,
and the same for all channels.

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