drm/amdgpu: sync ce and me with SWITCH_BUFFER(2)

we used to adopt wait_reg_mem to let CE wait before DE finish page
updating, but from Tonga+, CE doesn't support wait_reg_mem package so
this logic no longer works.

so here is another approach to do same thing:
Insert two of SWITCH_BUFFER at both front and end of vm_flush can
guarantee that CE not go further to process IB_const before vm_flush
done.

Insert two of SWITCH_BUFFER also works on CI, so remove legency method
to sync CE and ME

v2:
Insert double SWITCH_BUFFER at front of vm flush as well.

Signed-off-by: monk.liu <monk.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>