drm/i915/skl: Use fb modifiers for sprites

While at it just outright remove the tiling check in
intel_check_sprite_plane because it's impossible: We only allow
untiled and X-tiled. This essentially reverts

commit 94c6419e
Author: Damien Lespiau <damien.lespiau@intel.com>
Date:   Mon Oct 29 15:14:51 2012 +0000

    drm/i915: Error out when trying to set a y-tiled as a sprite

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
[danvet: Drop the hunk in check_sprite, it's impossible.]
Cc: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>