drm: xlnx: zynqmp_dpsub: Add NULL check in zynqmp_audio_init

In the Linux kernel, the following vulnerability has been resolved: 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.