use asic's callback function get_xclk in amdgpu

v2: squash in removal of leftover debug info
(drm/amd/pp: Delete debug info in smu7_hwmgr.c) (Rex)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>