drm/amd/display: Populate dtbclk from bounding box

dtbclk is unavaliable from pmfw. Try to grab the value from bounding box

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Fangzhi Zuo <jerry.zuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>