drm/panel: Make drm_panel_init() static

drm/panel: Make drm_panel_init() static

Now that all panel drivers use devm_drm_panel_alloc(),
there are no external callers of drm_panel_init().
Make it static to prevent new users from bypassing the
refcounted allocation path.

Remove stale references to drm_panel_init() in kdocs.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20260508-drm_panel_init_rm-v2-10-0bd4ac429971@redhat.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>