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>

drivers/gpu/drm/i915/intel_audio.c

@@ -144,7 +144,7 @@ static uint32_t audio_config_setup_n_reg(int n, uint32_t val)
 
 /* check whether N/CTS/M need be set manually */
 static bool audio_rate_need_prog(struct intel_crtc *crtc,
-					struct drm_display_mode *mode)
+				 const struct drm_display_mode *mode)
 {
 	if (((mode->clock == TMDS_297M) ||
 		 (mode->clock == TMDS_296M)) &&