drm/i915: enable dip before writing data on gen4

While testing with the intel_infoframes tool on gen4, I see that when
video DIP is disabled, what we write to the DATA memory is not exactly
what we read back later.

This regression has been introduce in

commit 64a8fc01
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Thu Sep 22 11:16:00 2011 +0530

    drm/i915: fix ILK+ infoframe support

That commit was setting VIDEO_DIP_CTL to 0 when initializing, which
caused the problem.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=43947


Cc: stable@kernel.org
Tested-by: Yang Guang <guang.a.yang@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
[danvet: Pimped commit message by using the usual commit citation
layout.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>