drm: Clarify semantics of struct drm_mode_config.{min, max}_{width, height}

Add additional information on the semantics of the size fields in
struct drm_mode_config. Also add a TODO to review all driver for
correct usage of these fields.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211110103702.374-10-tzimmermann@suse.de