drm/amdgpu: Add comment to describe the purpose of one difficult if statement

Use == instead of != in the if statement to make code easier understood

Signed-off-by: Alex Xie <AlexBin.Xie@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>