drm/amdgpu: change read of GPU clock counter on Vega10 VF

Using unified VBIOS has performance drop in sriov environment.
The fix is switching to another register instead.

Signed-off-by: Eric Huang <JinhuiEric.Huang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>