drm/i915: Reject fence stride=0 on gen4+

Our checks for an invalid fence stride forgot to guard against
zero stride on gen4+. Fix it.

v2: Avoid duplicated code (danvet)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>