drm/vmwgfx: add proper framebuffer refcounting

Afact vmwgfx already has all the right refcounting implemented on the
backing storage, and we only need to ensure that the drm fb doesn't
disappear untimely. So holding onto the fb reference from _lookup
until vmw_kms_present has completed should be enough.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>