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>