drm/i915: Disable SR when more than one pipe is enabled

Self Refresh should be disabled on dual plane configs.  Otherwise, as
the SR watermark is not calculated for such configs, switching to non
VGA mode causes FIFO underrun and display flicker.

This fixes Korg Bug #14897.

Signed-off-by: David John <davidjon@xenontk.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: stable@kernel.org
Signed-off-by: Eric Anholt <eric@anholt.net>