drm/amd/display: Address coverity change

[Why]
Coverity picks up a defect with regards to array underflow.

[How]
Address coverity issue as recommended.

Reviewed-by: Leo Ma <hanghong.ma@amd.com>
Signed-off-by: Chris Park <chris.park@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>