drm/vc4: hdmi: Set the b-frame marker to the match ALSA's default.

ALSA's iec958 plugin by default sets the block start preamble
to 8, whilst this driver was programming the hardware to expect
0xF.
Amend the hardware config to match ALSA.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d0b126deb228baf1244c91e02ac0a8f7c9c60dc5.1599120059.git-series.maxime@cerno.tech