DRM/i915: Remove magic to prevent blank screen on gen4 chipsets

Since the root cause is understood now and with the fix

   commit 564ed191
   Author: Imre Deak <imre.deak@intel.com>
   Date:   Fri Jun 13 14:54:21 2014 +0300

       drm/i915: gmch: fix stuck primary plane due to memory self-refresh mode

in place the magic for G4x chipsets introduced with commit

   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.

to avoided occasional screen blanking on mode changes can finally
be removed.
It's been verified that Imre's fix also resolves the said issue.

Signed-off-by: Egbert Eich <eich@suse.de>
Tested-by: Stefan Dirsch <sndirsch@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>