drm/i915: Relax the sprite scaling limits checks

Reduce the size of the the src/dst viewport to keep the scalign ratios
in check.

v2: Below min size sprite handling squashed to previous patch

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>