drm/amd/display: Properly set u clock

[Why]
u clk set request was being sent in units of mts, when it needed to be
in units of Mhz

[How]
add a division by 16 to convert from mts to Mhz

Signed-off-by: Aidan Wood <Aidan.Wood@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>