drm/vmwgfx: Turn on DRIVER_ATOMIC flag

Now that the legacy path has been tested, turn on the
DRIVER_ATOMIC flag so user mode driver can start going through
the Atomic path.

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>