drm/vc4: Disable overrun interrupts

We have a read-modify-write race when updating SCALER_DISPCTRL for
underrun and end-of-frame interrupts.
Ideally it would be fixed via a spinlock or similar, but that will
require a reasonable amount of study to ensure we don't get deadlocks.

The underrun reporting is only for debug, so disable it for now.

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