drm/i915: Turn on 945 self-refresh only if single CRTC is active

Enable self-refresh on 945 when just one CRTC is activated.
Otherwise user would get display flicker with dual display.

This fixes https://bugs.freedesktop.org/show_bug.cgi?id=27667



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