uapi/drm/drm_fourcc.h: Note on platform specificity for format modifiers

Make an additional note on DRM format modifiers for x and y tiling. These
format modifiers are defined for BDW+ platforms and therefore definition
is not valid for older gens. This is due to address swizzling for tiled
surfaces is no longer used. For newer platforms main memory controller has
a more effective address swizzling algorithm.

v2: Rephrase comment (Daniel)

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200506120827.12250-1-mika.kahola@intel.com