drm/amdgpu/uvd7: optimize uvd initialization sequence for SRIOV

1.Since in sriov there is no need of decoding, so skip the related code;
2.Vcpu boot up and umc enable need to take at the end of the init sequence;

Signed-off-by: Frank Min <Frank.Min@amd.com>
Signed-off-by: Xiangliang.Yu <Xiangliang.Yu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>