The newly added sound driver depends on SND_SOC_HDMI_CODEC, which in
turn only makes sense when ASoC is enabled, as shown by this warning:

warning: (DRM_MSM && DRM_STI && DRM_MEDIATEK_HDMI && DRM_I2C_NXP_TDA998X && DRM_DW_HDMI_I2S_AUDIO) selects SND_SOC_HDMI_CODEC which has unmet direct dependencies (SOUND && !M68K && !UML && SND && SND_SOC)

Since the audio driver is probably useless without the audio subsystem,
adding a dependency here seems the right solution.

Fixes: 2761ba6c ("drm: bridge: add DesignWare HDMI I2S audio support")
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20161125205411.1157522-1-arnd@arndb.de