drm/radeon: use normal BOs for the page tables v4

No need to make it more complicated than necessary,
just allocate the page tables as normal BO and
flush whenever the address change.

v2: update comments and function name
v3: squash bug fixes, page directory and tables patch
v4: rebased on Mareks changes

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