drm: document DRM_IOCTL_MODE_GETFB2

There are a few details specific to the GETFB2 IOCTL.

It's not immediately clear how user-space should check for the
number of planes. Suggest using the handles field or the pitches
field.

The modifier array is filled with zeroes, ie. DRM_FORMAT_MOD_LINEAR.
So explicitly tell user-space to not look at it unless the flag is
set.

Changes in v2 (Daniel):
- Mention that handles should be used to compute the number of planes,
  and only refer to pitches as a fallback.
- Reword bit about undefined modifier.

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211123112400.22245-1-contact@emersion.fr