drm/i915: Drop vblank wait from intel_dp_link_down

Nothing in Bspec seems to indicate that we actually needs this, and it
looks like can't work since by this point the pipe is off and so
vblanks won't really happen any more.

Note that Bspec mentions that it takes a vblank for this bit to
change, but _only_ when enabling.

Dropping this code quenches an annoying backtrace introduced by the
more anal checking since

commit 51e31d49
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Sep 15 12:36:02 2014 +0200

    drm/i915: Use generic vblank wait

Note: This fixes the fallout from the above commit, but does not address
the shortcomings of the IBX transcoder select workaround implementation
discussed during review [1].

[1] http://mid.gmane.org/87y4o7usxf.fsf@intel.com

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=86095


Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: stable@vger.kernel.org # 3.19
Signed-off-by: Jani Nikula <jani.nikula@intel.com>