drm/radeon/radeon_audio: Move 'dce6_*()'s prototypes to shared header

drm/radeon/radeon_audio: Move 'dce6_*()'s prototypes to shared header

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

 drivers/gpu/drm/radeon/dce6_afmt.c:32:5: warning: no previous prototype for ‘dce6_endpoint_rreg’ [-Wmissing-prototypes]
 32 | u32 dce6_endpoint_rreg(struct radeon_device *rdev,
 | ^~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/dce6_afmt.c:46:6: warning: no previous prototype for ‘dce6_endpoint_wreg’ [-Wmissing-prototypes]
 46 | void dce6_endpoint_wreg(struct radeon_device *rdev,
 | ^~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/dce6_afmt.c:110:6: warning: no previous prototype for ‘dce6_afmt_select_pin’ [-Wmissing-prototypes]
 110 | void dce6_afmt_select_pin(struct drm_encoder *encoder)
 | ^~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/dce6_afmt.c:123:6: warning: no previous prototype for ‘dce6_afmt_write_latency_fields’ [-Wmissing-prototypes]
 123 | void dce6_afmt_write_latency_fields(struct drm_encoder *encoder,
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/dce6_afmt.c:152:6: warning: no previous prototype for ‘dce6_afmt_hdmi_write_speaker_allocation’ [-Wmissing-prototypes]
 152 | void dce6_afmt_hdmi_write_speaker_allocation(struct drm_encoder *encoder,
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/dce6_afmt.c:177:6: warning: no previous prototype for ‘dce6_afmt_dp_write_speaker_allocation’ [-Wmissing-prototypes]
 177 | void dce6_afmt_dp_write_speaker_allocation(struct drm_encoder *encoder,
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/dce6_afmt.c:202:6: warning: no previous prototype for ‘dce6_afmt_write_sad_regs’ [-Wmissing-prototypes]
 202 | void dce6_afmt_write_sad_regs(struct drm_encoder *encoder,
 | ^~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/dce6_afmt.c:268:6: warning: no previous prototype for ‘dce6_hdmi_audio_set_dto’ [-Wmissing-prototypes]
 268 | void dce6_hdmi_audio_set_dto(struct radeon_device *rdev,
 | ^~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/dce6_afmt.c:287:6: warning: no previous prototype for ‘dce6_dp_audio_set_dto’ [-Wmissing-prototypes]
 287 | void dce6_dp_audio_set_dto(struct radeo_device *rdev,
 | ^~~~~~~~~~~~~~~~~~~~~

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>