drm/i915: Fix OGLC performance regression on 945

He Shuang reported an OGLC performance regression introduced in the patch
"enable memory self refresh on 9xx", In that patch, SR on 945 is disabled
everytime when calling intel_mark_busy(), while too much of such operation
will impact performance. Actually disable SR is necessary only when GPU and
Crtc changing from idle to busy. This patch make such optimization.

It fixes upstream bug
http://bugs.freedesktop.org/show_bug.cgi?id=26422



Signed-off-by: Li Peng <peng.li@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>