drm/amdgpu: wait on page directory changes. v2

Pagetables can be moved and therefore the page directory update can be necessary
for the current cs even if none of the the bo's are moved. In that scenario
there is no fence between the sdma0 and gfx ring, so we add one.

v2 (chk): rebased

Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>