drm/i915: wait for a vblank to pass after tv detect

Otherwise the hw will get confused and result in a black screen.

This regression has been most likely introduce in

commit 974b9331
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Sun Sep 5 00:44:20 2010 +0100

    drm/i915/tv: Poll for DAC state change

That commit replace the first msleep(20) with a busy-loop, but failed
to keep the 2nd msleep around. Later on we've replaced all these
msleep(20) by proper vblanks.

For reference also see the commit in xf86-video-intel:

commit 1142be53eb8d2ee8a9b60ace5d49f0ba27332275
Author: Jesse Barnes <jbarnes@hobbes.lan>
Date:   Mon Jun 9 08:52:59 2008 -0700

    Fix TV programming:  add vblank wait after TV_CTL writes

    Fxies FDO bug #14000; we need to wait for vblank after
    writing TV_CTL or following "DPMS on" calls may not actually enable the output.

v2: As suggested by Chris Wilson, add a small comment to ensure that
no one accidentally removes this vblank wait again - there really
seems to be no sane explanation for why we need it, but it is
required.

Launchpad: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/763688


Reported-and-Tested-by: Robert Lowery <rglowery@exemail.com.au>
Cc: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Cc: stable@vger.kernel.org
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>