drm/amd/pp: Fix truncated clock value when set watermark

the clk value should be tranferred to MHz first and
then transfer to uint16. otherwise, the clock value
will be truncated.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reported-by: Hersen Wu <hersenxs.wu@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org