drm/amdgpu: Make amdgpu_ring_mux functions as static

lkp robot reported missing-prototypes and unused-but-set-variable warnings on
some functions of amdgpu_mcbp_mux.c. Make them static and remove the unused
variable.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Jiadong Zhu <Jiadong.Zhu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>