drm/i915/skl: Add new displayable tiling formats

Starting with SKL display engine can scan out Y, and newly introduced Yf
tiling formats so add the latter to the frame buffer modifier space.

v2: Definitions moved to drm_fourcc.h.
v3: Try to document the format better.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>