drm/vmwgfx: Make fence sequences continous across a VT switch.

A vt switch in stealth mode would take down the FIFO, and re-
initialize fence sequence numbers. This patch
saves the current state of the fence sequence when the FIFO is
disabled.

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