drm/vc4: hdmi: Avoid full hdmi audio fifo writes

drm/vc4: hdmi: Avoid full hdmi audio fifo writes

[ Upstream commit 1c594eeccf92368177c2e22f1d3ee4933dfb8567 ]

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

Fixes: bb7d78568814 ("drm/vc4: Add HDMI audio support")
Signed-off-by: Dom Cobley <popcornmix@gmail.com>
Link: https://lore.kernel.org/r/20220613144800.326124-21-maxime@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Sasha Levin <sashal@kernel.org>