drm: extract closefb logic in separate function

drm_mode_rmfb performs two operations: drop the FB from the
file_priv->fbs list, and make sure the FB is no longer used on a
plane.

In the next commit an IOCTL which only does so former will be
introduced, so let's split it into a separate function.

No functional change, only refactoring.

v2: no change

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Dennis Filder <d.filder@web.de>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231020101926.145327-1-contact@emersion.fr