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

 drivers/gpu/drm/radeon/dce3_1_afmt.c:30:6: warning: no previous prototype for ‘dce3_2_afmt_hdmi_write_speaker_allocation’ [-Wmissing-prototypes]
 30 | void dce3_2_afmt_hdmi_write_speaker_allocation(struct drm_encoder *encoder,
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/dce3_1_afmt.c:48:6: warning: no previous prototype for ‘dce3_2_afmt_dp_write_speaker_allocation’ [-Wmissing-prototypes]
 48 | void dce3_2_afmt_dp_write_speaker_allocation(struct drm_encoder *encoder,
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/dce3_1_afmt.c:66:6: warning: no previous prototype for ‘dce3_2_afmt_write_sad_regs’ [-Wmissing-prototypes]
 66 | void dce3_2_afmt_write_sad_regs(struct drm_encoder *encoder,
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/dce3_1_afmt.c:116:6: warning: no previous prototype for ‘dce3_2_audio_set_dto’ [-Wmissing-prototypes]
 116 | void dce3_2_audio_set_dto(struct radeon_device *rdev,
 | ^~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/dce3_1_afmt.c:170:6: warning: no previous prototype for ‘dce3_2_hdmi_update_acr’ [-Wmissing-prototypes]
 170 | void dce3_2_hdmi_update_acr(struct drm_encoder *encoder, long offset,
 | ^~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/dce3_1_afmt.c:202:6: warning: no previous prototype for ‘dce3_2_set_audio_packet’ [-Wmissing-prototypes]
 202 | void dce3_2_set_audio_packet(struct drm_encoder *encoder, u32 offset)
 | ^~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/dce3_1_afmt.c:223:6: warning: no previous prototype for ‘dce3_2_set_mute’ [-Wmissing-prototypes]
 223 | void dce3_2_set_mute(struct drm_encoder *encoder, u32 offset, bool mute)
 | ^~~~~~~~~~~~~~~

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: Slava Grigorev <slava.grigorev@amd.com>
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>