drm/i915/bxt: fix WaForceContextSaveRestoreNonCoherent on steppings B0+

On B0 and C0 steppings the workaround enable bit would be overriden by
default, so the overriding must be disabled.

The WA was added in
commit 83a24979


Author: Nick Hoath <nicholas.hoath@intel.com>
Date:   Fri Apr 10 13:12:26 2015 +0100

    drm/i915/bxt: Add WaForceContextSaveRestoreNonCoherent

Spotted-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>