In some case vblank might not be initialized and we shouldn't
try to use associated function. This patch make sure this is
the case. It also export drm_vblank_cleanup so driver can cleanup
vblank if for any reason IRQ/MSI is not working.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>