drm/i915/skl: don't set the AsyncFlip performance mode for Gen9+

drm/i915/skl: don't set the AsyncFlip performance mode for Gen9+

The following sets the AsyncFlip performance mode for everything above
Gen6:

commit 4790cb36b3eede8fb0cca529dc1d31b9936fa24b
Author: Chris Wilson <[email protected]>
Date:   Sun Jan 20 16:11:20 2013 +0000

    drm/i915: Disable AsyncFlip performance optimisations

Starting from Gen9 the MI_MODE register layout changes and doesn't
include the above bit.

Reviewed-by: Thomas Wood <[email protected]>
Signed-off-by: Imre Deak <[email protected]>
Signed-off-by: Damien Lespiau <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>