drm/vc4: hdmi: Deal with multiple ALSA cards

The HDMI driver was registering a single ALSA card so far with the name
vc4-hdmi.

Obviously, this is not going to work anymore when we will have multiple
HDMI controllers since we will end up trying to register two files with the
same name.

Let's use the variant to avoid that name conflict.

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>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e60a37444e848a384a45707a21d6df8883115f86.1599120059.git-series.maxime@cerno.tech