drm/amd/display: Fix array-index-out-of-bounds in dml2

[Why]
UBSAN errors observed in dmesg.
array-index-out-of-bounds in dml2/display_mode_core.c

[How]
Fix the index.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>