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

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

i915's fbdev contains code for restoring the client's framebuffer. It
is specific to i195 and cannot be ported to the common fbdev client.

Introduce the callback struct drm_fb_helper.fb_restore and implement
it for i915. The fbdev helpers invoke the callback after restoring the
fbdev client.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241212170913.185939-6-tzimmermann@suse.de
Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>