drm/amdgpu: fix array out of bounds

When the initial value of i is greater than zero,
it may cause endless loop, resulting in array out
of bounds, fix it.

This is a port of the radeon fix to amdgpu.

Signed-off-by: tom will <os@iscas.ac.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>