drm/amdgpu: Update PDEs flush TLB if PTB/PDB moved

Flush TLBs when existing PDEs are updated because a PTB or PDB moved,
but avoids unnecessary TLB flushes when new PDBs or PTBs are added to
the page table, which commonly happens when memory is mapped for the
first time.

Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>