drm/i915: Remove the WaDisableBackToBackFlipFix w/a for Haswell

This workaround is only valid for IVB and VLV and the write triggers an
error on HSW.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanonI@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>