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.