Xe2 CCS modifiers have undocumented requirements

[Chery, Nanley G]

Hi,

While reviewing Mesa's modifier implementation, I discovered that the kernel carries forward some requirements from gfx12:

- 16K alignment between planes (according to tgl_plane_min_alignment)
- 4 tiles wide (according to intel_fb_stride_alignment)

These requirements are not documented in drm_fourcc.h. Could we have them documented? But more importantly, are they required? I haven't seen anything in the Bspec to suggest so.

BTW, I reported this issue in the Xe gitlab [1], but I'm reposting here for visibility.

-Nanley

1. https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/3679