vmwgfx: Fix display system init & close functions

Make sure we null the display private, make sure we catch and
handle vblank failing to init and don't call vblank_cleanup if
we haven't initialized the display system.

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