Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/radeon/evergreen_hdmi.c:37:6: warning: no previous prototype for ‘dce4_audio_enable’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/evergreen_hdmi.c:67:6: warning: no previous prototype for ‘evergreen_hdmi_update_acr’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/evergreen_hdmi.c:97:6: warning: no previous prototype for ‘dce4_afmt_write_latency_fields’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/evergreen_hdmi.c:119:6: warning: no previous prototype for ‘dce4_afmt_hdmi_write_speaker_allocation’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/evergreen_hdmi.c:137:6: warning: no previous prototype for ‘dce4_afmt_dp_write_speaker_allocation’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/evergreen_hdmi.c:155:6: warning: no previous prototype for ‘evergreen_hdmi_write_sad_regs’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/evergreen_hdmi.c:208:6: warning: no previous prototype for ‘evergreen_set_avi_packet’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/evergreen_hdmi.c:227:6: warning: no previous prototype for ‘dce4_hdmi_audio_set_dto’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/evergreen_hdmi.c:270:6: warning: no previous prototype for ‘dce4_dp_audio_set_dto’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/evergreen_hdmi.c:306:6: warning: no previous prototype for ‘dce4_set_vbi_packet’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/evergreen_hdmi.c:317:6: warning: no previous prototype for ‘dce4_hdmi_set_color_depth’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/evergreen_hdmi.c:354:6: warning: no previous prototype for ‘dce4_set_audio_packet’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/evergreen_hdmi.c:389:6: warning: no previous prototype for ‘dce4_set_mute’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/evergreen_hdmi.c:400:6: warning: no previous prototype for ‘evergreen_hdmi_enable’ [-Wmissing-prototypes]
 drivers/gpu/drm/radeon/evergreen_hdmi.c:440:6: warning: no previous prototype for ‘evergreen_dp_enable’ [-Wmissing-prototypes]

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>