drm/vmwgfx/ttm: convert vram mm init to new code paths

Split out the vram thp init path vs the range manager init.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-21-airlied@gmail.com