drm/vmwgfx: Use fb handover mechanism instead of stealth mode.

When the vmwgfx module is loaded on top of vesafb, it would operate in
stealth mode in parallel with vesafb, evicting VRAM on dropmaster.

Change that to use the vesafb handover mechanism, like other drmfb drivers.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>