drm/radeon: fence virtual address and free it once idle v4

Virtual address need to be fenced to know when we can safely remove it.
This patch also properly clear the pagetable. Previously it was
serouisly broken.

Kernel 3.5/3.4 need a similar patch but adapted for difference in mutex locking.

v2: For to update pagetable when unbinding bo (don't bailout if
    bo_va->valid is true).
v3: Add kernel 3.5/3.4 comment.
v4: Fix compilation warnings.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>