drm/amdgpu:fix get wrong gfx always on cu masks.

Bug: SWDEV-117987: Always on CU mask broken for gfx7+

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>