drm/amdgpu: set adev->num_vmhubs for gmc6,7,8

So that we properly handle them on older asics.

Fixes: 3ff98548 ("drm/amdgpu: Export function to flush TLB of specific vm hub")
Tested-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>