drm/i915: treat src w & h as fixed point in sprite handling code

This was missed when we converted the source values to 16.16 fixed point.

Cc: stable@vger.kernel.org
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>