drm/i915: Add support for sprites on g4x

Now that the watermarks are in order, it should be safe to enable sprite
planes on g4x. We alreday have the code in fact, we just call it ilk_.
Let's rename to g4x_ and let it loose.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170421181432.15216-16-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>