drm/cirrus: Invalidate page tables when pinning a BO

This is a cirrus version of Egbert Eich's patch for mgag200.

Without bo.bdev->dev_mapping set, the ttm_bo_unmap_virtual_locked
called from ttm_bo_handle_move_mem returns with no effect. If any
application accessed the memory before it was moved, it will
access wrong memory next time. This causes crashes when changing
resolution down.

Signed-off-by: Michal Srb <msrb@suse.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>