drm/radeon/radeon_audio: Move 'r600_*' prototypes into shared header

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

 drivers/gpu/drm/radeon/r600_hdmi.c:177:6: warning: no previous prototype for ‘r600_hdmi_update_acr’ [-Wmissing-prototypes]
 177 | void r600_hdmi_update_acr(struct drm_encoder *encoder, long offset,
 | ^~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/r600_hdmi.c:217:6: warning: no previous prototype for ‘r600_set_avi_packet’ [-Wmissing-prototypes]
 217 | void r600_set_avi_packet(struct radeon_device *rdev, u32 offset,
 | ^~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/r600_hdmi.c:314:6: warning: no previous prototype for ‘r600_hdmi_audio_set_dto’ [-Wmissing-prototypes]
 314 | void r600_hdmi_audio_set_dto(struct radeon_device *rdev,
 | ^~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/r600_hdmi.c:340:6: warning: no previous prototype for ‘r600_set_vbi_packet’ [-Wmissing-prototypes]
 340 | void r600_set_vbi_packet(struct drm_encoder *encoder, u32 offset)
 | ^~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/r600_hdmi.c:351:6: warning: no previous prototype for ‘r600_set_audio_packet’ [-Wmissing-prototypes]
 351 | void r600_set_audio_packet(struct drm_encoder *encoder, u32 offset)
 | ^~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/r600_hdmi.c:393:6: warning: no previous prototype for ‘r600_set_mute’ [-Wmissing-prototypes]
 393 | void r600_set_mute(struct drm_encoder *encoder, u32 offset, bool mute)
 | ^~~~~~~~~~~~~
 drivers/gpu/drm/radeon/r600_hdmi.c:469:6: warning: no previous prototype for ‘r600_hdmi_enable’ [-Wmissing-prototypes]
 469 | void r600_hdmi_enable(struct drm_encoder *encoder, bool enable)
 | ^~~~~~~~~~~~~~~~

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>