drm/amd/display: Handle Y carry-over in VCP X.Y calculation

[Why/How]
Theoretically rare corner case where ceil(Y) results in rounding
up to an integer. If this happens, the 1 should be carried over to
the X value.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>