drm/i915: Add missing const to audio_rate_need_prog()

The lack of const leads to a compile warning after merging i915
upstream tree:
   drivers/gpu/drm/i915/intel_audio.c:147:13: note: expected 'struct drm_display_mode *' but argument is of type 'const struct drm_display_mode *'

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Takashi Iwai <tiwai@suse.de>