drm/amdgpu: no need load microcode at sdma if powerplay is enabled

SDMA firmware will be loaded by SMU manager if powerplay is enabled.
So it needn't load at SDMA.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>