vc4/drm: Avoid full hdmi audio fifo writes

We are getting occasional VC4_HD_MAI_CTL_ERRORF in
HDMI_MAI_CTL which seem to correspond with audio dropouts.

Reduce the threshold where we deassert DREQ to avoid the fifo overfilling

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