drm: xlnx: zynqmp_dpsub: Add NULL check in zynqmp_audio_init

devm_kasprintf() calls can return null pointers on failure.
But some return values were not checked in zynqmp_audio_init().

Add NULL check in zynqmp_audio_init(), avoid referencing null
pointers in the subsequent code.

Fixes: 3ec5c157 ("drm: xlnx: zynqmp_dpsub: Add DP audio support")
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Charles Han <hanchunchao@inspur.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250211102049.6468-1-hanchunchao@inspur.com