drm/i915: Disable AsyncFlip performance optimisations

drm/i915: Disable AsyncFlip performance optimisations

commit 1c8c38c588ea91f8deeae21284840459d1bb58e3 upstream.

This is a required workarounds for all products, especially on gen6+
where it causes the command streamer to fail to parse instructions
following a WAIT_FOR_EVENT. We use WAIT_FOR_EVENT for synchronising
between the GPU and the display engines, and so this bit being unset may
cause hangs.

References: https://bugzilla.kernel.org/show_bug.cgi?id=52311
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[bwh: Backported to 3.2:
 - Adjust context
 - s/_MASKED_BIT/GFX_MODE/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>