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: NDavid John <davidjon@xenontk.org>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Cc: stable@kernel.org
Signed-off-by: NEric Anholt <eric@anholt.net>