drm/amd/display: Adding array index check to prevent memory corruption

[Why & How]
Array indices out of bound caused memory corruption. Adding checks to
ensure that array index stays in bound.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Leo Chen <leo.chen@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>