drm/amd/display: Round up DST_after_scaler to nearest int

[Why]
The DST_after_scaler value that DML spreadsheet outputs is
generally the driver value round up to the nearest int.

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Tested-by: Mark Broadworth <mark.broadworth@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>