drm/i915/skl: Framebuffers need to be aligned to 256KB on Skylake

v2: Also align X tiled fbs to 256KB (Thomas)

Reviewed-by: Thomas Wood <thomas.wood@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>