drm/i915: Do not set "Enable Panel Fitter" on SNB pageflips

Not only do the pageflip work without it at non-native modes (i.e. with
the panel fitter enabled), it also causes normal (non-pageflipped)
modesets to fail.

Reported-by: Adam Jackson <ajax@redhat.com>
Tested-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Wanted-by-for-fixes: Dave Airlie <airlied@gmail.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>