drm/amd/powerplay: Only load SDMA0/MEC firmware once on Stoney (v2)

Only load the SDMA0/MEC1 firmware once in the Carrizo SMU manager
driver.

(v2) Avoid loading SDMA0 twice too.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>