drm/amdgpu: add get_clock_info for atomfirmware

The information has moved to different tables, notably
smu_info for core refclk and umc_info for mem refclk.

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>