drm/doc: document drm_event and its types

Convert struct drm_event to a kernel doc comment. Link to the
generic DRM event types. Add a basic description of each event
type.

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20230717093032.600773-1-contact@emersion.fr