drm/amdgpu: call atomfirmware get_clock_info for atomfirmware systems

Rather than the legacy atombios version.

Acked-by: Chunming Zhou <david1.zhou@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>