drm/amdgpu: fix overflowed array index read warning

[ Upstream commit ebbc2ada5c636a6a63d8316a3408753768f5aa9f ]

Clear overflowed array index read warning by cast operation.

Signed-off-by: Tim Huang <Tim.Huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>