drm/i915: vlv: fix stuck primary plane due to SR watermarks

Blanking/unblanking the console in a loop on an Asus T100 sometimes
leaves the console blank. After some digging I found that applying

commit 61bc95c1fbbb6a08b55bbe161fdf1ea5493fc595
Author: Egbert Eich <eich@suse.com>
Date:   Mon Mar 4 09:24:38 2013 -0500

    DRM/i915: On G45 enable cursor plane briefly after enabling the display plane.

fixed VLV too. At least in my case the problem seems to happen already
during the previous crtc disabling, and it goes away if we disable SR
watermarks before disabling the primary plane.

I also noticed that the problem only happens if the C7S CPU idle state
is entered, disabling that also gets rid of the problem. As an
alternative I also tried to increase the plane SR watermark close to its
maximum but that didn't solve the issue.

Signed-off-by: Imre Deak <imre.deak@intel.com>