drm/vmwgfx: Correctly set the enabled state on crtcs

Failure to do this would make the drm_mode_get_crtc ioctl return
without crtc mode info, indicating that no mode was set.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>