drm/amdgpu: Skip the program of MMMC_VM_AGP_* in SRIOV on MMHUB v3_0_0

[Why]
VF should not program these registers, the value were defined in the host.

[How]
Skip writing them in SRIOV environment and program them on host side.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Yifan Zha <Yifan.Zha@amd.com>
Signed-off-by: Horace Chen <horace.chen@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>