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

On G45 some low res modes (800x600 and 1024x768) produce a blank
screen when the display plane is enabled with with cursor plane
off.
Experiments showed that this issue occurred when the following
conditions were met:
a. a previous mode had the cursor plane enabled (Xserver).
b. this mode or the previous one was using self refresh. (Thus
   the problem was only seen with low res modes).
The screens lit up as soon as the cursor plane got enabled.
Therefore the blank screen occurred only in console mode, not
when running an Xserver.
It also seemed to be necessary to disable self refresh while briefly
enabling the cursor plane.

Signed-off-by: Egbert Eich <eich@suse.com>
Bugzilla: https://bugs.freedesktop.org/attachment.cgi?bugid=61457


Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: drop spurious whitespace change.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>