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>

include/uapi/drm/drm_fourcc.h

@@ -195,4 +195,19 @@
  */
 #define I915_FORMAT_MOD_Y_TILED	fourcc_mod_code(INTEL, 2)
 
+/*
+ * Intel Yf-tiling layout
+ *
+ * This is a tiled layout using 4Kb tiles in row-major layout.
+ * Within the tile pixels are laid out in 16 256 byte units / sub-tiles which
+ * are arranged in four groups (two wide, two high) with column-major layout.
+ * Each group therefore consits out of four 256 byte units, which are also laid
+ * out as 2x2 column-major.
+ * 256 byte units are made out of four 64 byte blocks of pixels, producing
+ * either a square block or a 2:1 unit.
+ * 64 byte blocks of pixels contain four pixel rows of 16 bytes, where the width
+ * in pixel depends on the pixel depth.
+ */
+#define I915_FORMAT_MOD_Yf_TILED fourcc_mod_code(INTEL, 3)
+
 #endif /* DRM_FOURCC_H */