This is a requirement added to the spec. This patch will prevent
persistent corruption on the display.

v2: Make the wait before the vblank wait. (Art)
Try to finish early by polling the register
s/present/prevent (Chris)

Cc: Art Runyan <arthur.j.runyan@intel.com>
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
[danvet: Upgrade debug output to ERROR.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>