drm/client: Move client event handlers to drm_client_event.c

A number of DRM-client functions serve as entry points from device
operations to client code. Moving them info a separate file will later
allow for a more fine-grained kernel configuration. For most of the
users it is sufficient to include <drm/drm_client_event.h> instead of
the full driver-side interface in <drm/drm_client.h>

v2:
- rename new files to drm_client_event.{c,h}

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tursulin@ursulin.net>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Danilo Krummrich <dakr@redhat.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241014085740.582287-7-tzimmermann@suse.de