drm/i915: disable VGA plane reliably

This does VGA disable like DDX driver. SR01 bit 5 should be set
before VGA plane disable through control register, otherwise we
might get random crash and lockups.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
1 file changed