drm/vmwgfx: switch to own vma manager

Add struct drm_vma_offset_manager to vma_private, initialize it and
pass it to ttm_bo_device_init().

With this in place the last user of ttm's embedded vma offset manager
is gone and we can remove it (in a separate patch).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Thomas Hellström <thellstrom@vmware.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190905070509.22407-8-kraxel@redhat.com