drm/amd/display: Fix comparison error in dcn21 DML
[why]
A comparison error made it possible to not iterate through all the
specified prefetch modes.
[how]
Correct "<" to "<="
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com> Reviewed-by: Yongqiang Sun <Yongqiang.Sun@amd.com> Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com> Signed-off-by: Victor Lu <victorchengchi.lu@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>