drm/i915/display: fbdev: Move custom suspend code to new callback

drm/i915/display: fbdev: Move custom suspend code to new callback

If the fbdev buffer is backed by stolen memory, it has to be cleared
upon resume from hibernation. Move the code into the new callback
fb_set_suspend, so that it can run from DRM's generic fbdev client.
No functional change. Other drivers are not affected.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241212170913.185939-7-tzimmermann@suse.de
Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>