drm/i915/uapi: Add comment clarifying purpose of I915_TILING_* values

The I915_TILING_* values in our uapi header are intended solely for use
with the old get_tiling/set_tiling ioctls that operate on hardware
de-tiling fences; all other uapi communication about tiling types is
done via framebuffer modifiers rather than with these old values.

On newer Intel platforms detiling fences no longer exist so the old
get_tiling/set_tiling ioctls are no longer usable and will always return
-EOPNOTSUPP.  This means there's no reason to add new tiling types (such
as the Tile4 format introduced by Xe_HP) to the uapi header here.  Any
kernel-internal code that needs to represent tiling format should either
rely on framebuffer modifiers (as the display code does) or use some
kind of non-uapi enum (as the GEM blt selftest now does).

References: https://patchwork.freedesktop.org/patch/456656/?series=95308
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Caz Yokoyama <caz.yokoyama@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211012221245.2609670-1-matthew.d.roper@intel.com

include/uapi/drm/i915_drm.h

@@ -1522,6 +1522,12 @@ struct drm_i915_gem_caching {
 #define I915_TILING_NONE	0
 #define I915_TILING_X		1
 #define I915_TILING_Y		2
+/*
+ * Do not add new tiling types here.  The I915_TILING_* values are for
+ * de-tiling fence registers that no longer exist on modern platforms.  Although
+ * the hardware may support new types of tiling in general (e.g., Tile4), we
+ * do not need to add them to the uapi that is specific to now-defunct ioctls.
+ */
 #define I915_TILING_LAST	I915_TILING_Y
 
 #define I915_BIT_6_SWIZZLE_NONE		0