drm/amdgpu: add amdgpu_virt_get_vf_mode helper function

the swsmu or powerplay(hwmgr) need to handle task according to different VF mode,
this function to help query vf mode.

vf mode:
1. SRIOV_VF_MODE_BARE_METAL: the driver work on host  OS (PF)
2. SRIOV_VF_MODE_ONE_VF    : the driver work on guest OS with one VF
3. SRIOV_VF_MODE_MULTI_VF  : the driver work on guest OS with multi VF

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>