drm/i915: Reorganize sprite init

Kill the switch statement from the sprite init code and replace with a
more straightforward if ladder. Now each significant evolution of the
sprite hardware is in its own neat box.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477411083-19255-5-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>