drm/amdgpu: Optimize a function called by every IB sheduling

  Move several if statements and a loop statment from
  run time to initialization time.

Signed-off-by: Alex Xie <AlexBin.Xie@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>