drivers: gpu: Mark function as static in r600_hdmi.c

Mark function r600_audio_set_dto() as static in drm/radeon/r600_hdmi.c
because it is not used outside this file.

This eliminates the following warning in drm/radeon/r600_hdmi.c:
drivers/gpu/drm/radeon/r600_hdmi.c:253:6: warning: no previous prototype for ‘r600_audio_set_dto’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>