drm: document struct drm_mode_fb_cmd2

Follow-up for the DRM_IOCTL_MODE_GETFB2 docs.

v2: (Daniel Stone)
- Replace fourcc.org with drm_fourcc.h because this is the
  authoritative source and the website may have mismatches.
- Drop assumption that offsets will generally be 0.
- Mention that unused entries must be zero'ed out.

v3: (Pekka)
- Mention that a handle can be re-used
- Add unit for pitches/offsets

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Daniel Stone <daniels@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220203133753.261507-1-contact@emersion.fr