drm/amd/amdgpu: Clean up header file of symbols that are defined to be static

[Why]
Header file exports functions get_gpu_clock_counter(), get_cu_info() and
select_se_sh() that are defined to be static

Signed-off-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>