drm/amdgpu: move adjust adjust_mc_addr into the GFX9 vm_flush functions

That GFX9 needs a PDE in the registers is entirely GFX9 specific.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>