drm: xlnx: zynqmp_dpsub: fix Kconfig dependencies for ASoC

The new audio code fails to build when sounds support is in a loadable
module but the GPU driver is built-in:

x86_64-linux-ld: zynqmp_dp_audio.c:(.text+0x6a8): undefined reference to `devm_snd_soc_register_card'
x86_64-linux-ld: drivers/gpu/drm/xlnx/zynqmp_dp_audio.o:(.rodata+0x1bc): undefined reference to `snd_soc_info_volsw'
x86_64-linux-ld: drivers/gpu/drm/xlnx/zynqmp_dp_audio.o:(.rodata+0x1f0): undefined reference to `snd_soc_get_volsw'
x86_64-linux-ld: drivers/gpu/drm/xlnx/zynqmp_dp_audio.o:(.rodata+0x1f4): undefined reference to `snd_soc_put_volsw'

Change the Kconfig dependency to disallow the sound support in this
configuration.

Fixes: 3ec5c1579305 ("drm: xlnx: zynqmp_dpsub: Add DP audio support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250227132036.1136600-1-arnd@kernel.org