drm/i915: Remove the rounding down of the gen4+ fence region

Restricting the fence to the end of the previous tile-row breaks access
to the final portion of the object. On gen2/3 we employed lazy fencing
to pad out the fence with scratch page to provide access to the tail,
and now we also pad out the object on gen4+ we can apply the same fix.

Fixes: af1a7301 ("drm/i915: Only fence tiled region of object.")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170109161613.11881-5-chris@chris-wilson.co.uk