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

commit 61bc95c1
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.

In my case the problem seemed to happen already during the previous crtc
disabling and went away if I disabled self-refresh mode before disabling
the primary plane.

The root cause for this is that updates from the shadow to live plane
control register are blocked at vblank time if the memory self-refresh
mode (aka max-fifo mode on VLV) is active at that moment. The controller
checks at frame start time if the CPU is in C0 and the self-refresh mode
enable bit is set and if so activates self-reresh mode, otherwise
deactivates it. So to make sure that the plane truly gets disabled before
pipe-off we have to:

1. disable memory self-refresh mode
2. disable plane
3. wait for vblank
4. disable pipe
5. wait for pipe-off

v2:
- add explanation for the root cause from HW team (Cesar Mancini et al)
- remove note about the CPU C7S state, in my latest tests disabling it
  alone didn't make a difference
- add vblank between disabling plane and pipe (Ville)
- apply the same workaround for all gmch platforms (Ville)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Reviewed-by: Deepak S<deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>