drm/radeon: remove unused declarations for radeon

radeon_combios_get_ext_tmds_info() has been removed since
commit fcec570b ("drm/radeon/kms: add support for
external tmds on legacy boards").

radeon_mst has been removed since
commit 01ad1d9c ("drm/radeon: Drop legacy MST support").

r600_hdmi_acr() has been removed since
commit 64424d6e ("radeon/audio: consolidate update_acr()
functions (v2)").

So remove the declarations for them from header file.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220913024847.552254-4-cuigaosheng1@huawei.com