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>