drm/fb-helper: Document struct drm_fb_helper.hint_leak_smem_start

Document the new field smem_start in struct drm_fb_helper and avoid
a compile-time warning. An error message is shown below and the bug
report is at [1].

  include/drm/drm_fb_helper.h:204: warning: Function parameter or member 'hint_leak_smem_start' not described in 'drm_fb_helper'

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: e7c5c29a ("drm/fb-helper: Set flag in struct drm_fb_helper for leaking physical addresses")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Link: https://lore.kernel.org/dri-devel/20221107143858.0253a8ff@canb.auug.org.au/T/#u # [1]
Link: https://patchwork.freedesktop.org/patch/msgid/20221107125329.12842-4-tzimmermann@suse.de