drm/vmwgfx: fix regression in thp code due to ttm init refactor.

When I refactored this code with the new init paths, I failed to
set the funcs back up properly, this caused a failure to bringup
gdm properly.

Fixes: 252f8d7b

 ("drm/vmwgfx/ttm: convert vram mm init to new code paths")
Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201001042012.13114-1-airlied@gmail.com