drm: Avoid calling vblank function is vblank wasn't initialized

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>