drm/amd/display: Halve DTB Clock Value for DCN32

VBIOS default clock value was halved, so the hardcoded dtb value should be
halved as well.

dtb clock should come from SMU eventually, but now dtb clock switching is not
fully supported yet in SMU.

Halve the dtb hardcoded value for now to have UHBR10 light up. Will rely on
SMU for dtb clock switching once available. The w/a is for DCN32 only, DCN321
should adopt the original value.

Signed-off-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>