drm/amd/display: Limit nv21 dst_y

Dst_y can become negative in extreme odm 4to1 cases. While not strictly
invalid, this should be limited to 0 for rq/dlg/ttu calculation.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>