In drm_wait_one_vblank() add a delay when we're "cold". Interrupts aren't

In drm_wait_one_vblank() add a delay when we're "cold".  Interrupts aren't
enabled at that point, so we cannot wait for one to happen.  But having no
delay at all breaks detection of some output connectors.

Thanks to Philippe Meunier for tracking down the issue.

ok millert@, jsg@