drm/i915: Set framebuffer alignment based upon the fence constraints.

Set the request alignment to 0, and leave it up to i915_gem_object_pin()
to set the appropriate alignment to match the fence covering the object.

Eric Anholt mentioned that the pinning code is meant to choose the
maximum of the request alignment and that of the fence covering the
object... However currently, the pinning code will only apply the fence
constraints if the supplied alignment is 0.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>