drm/amdgpu: Initialize the variables in a straight-forward way

Initialize the variable in a straight-forward way instead of
hiding the initialization inside the loop. This can also
reduce one function call.

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>