drm/i915: fix pipe source image setting in flip command

The MI_DISPLAY_FLIP command needs to be set the same pipe
source image like in pipe source register, e.g source image
size minus one. This fixes screen corrupt issue on Ironlake.

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