drm/amdgpu: Export function to flush TLB of specific vm hub

This is for kfd to reuse amdgpu TLB invalidation function.
On gfx10, kfd only needs to flush TLB on gfx hub but not
on mm hub. So export a function for KFD flush TLB only on
specific hub.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Christian Konig <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>