drm/i915: Ivybridge is the odd one when it comes to pipe scalers

Between ivb, hsw and vlv, only Ivybridge has sprites with scaling
capabilities.

Also make max_downscale coherent with that.

v2: Rebase on top of the recent ivb/vlv/hsw sprite scaling fixes.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com> (v1)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>