ALSA: hda - Reduce ifdef CONFIG_SND_HDA_I915

We're providing dummy functions for CONFIG_SND_HDA_i915=n, thus ifdef
can be reduced.  (But hda_i915_init() has to be fixed to return zero.)

This automatically fixes a compile warning:
  sound/pci/hda/hda_intel.c: In function 'azx_probe_continue':
  sound/pci/hda/hda_intel.c:1966:1: warning: label 'i915_power_fail' defined but not used [-Wunused-label]

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>